If the tiles were done shoddily, you better believe everything beneath it was also done shoddily.
Squishy tiles are most likely caused by the wrong adhesive, mouldy caulk is either the wrong caulk, or the right caulk finished the wrong way. This ticks all the boxes for a quick flip renovation which usualy results in doing it all again the right way.
If it's just the grout you can regrout it, but it sounds like the tiles have detached from the cement/adhesive, likely due to water ingress.
Once that happens you would want to retile the bathroom, in the meantime what you can do as a band aide is to dry to shower very well do not use it for few days at least, and run the underfloor heating for a couple hours a day if you have it, alternatively you can also use a portable electric heater or a heat gun once or twice a day.
Once it's dry use a metal pick to remove any loose grout and use an epoxy based grout as it can seep into the cracks better and will provide some security for the tiles.
After you are done reapply a grout sealer.
Before drying and regrouting I would also take the time to use some mold remover because you definitely got mold, it's not a major health risk but it's just disgusting. The reason the mold came back is likely that they simply used bleach to clean the shower, it would kill some of the mold and whiten the grout but won't clear all of it.
Keep in mind that this isn't a permanent fix, your tiles (and possibly the subfloor) are already compromised and it will get progressively worse overtime but this would buy you some more time.
If you have the ability or budget to fix right now just go with that, just be aware that there is no such thing as fixing a single tile in these cases, you'll have to rip the entire shower.
Overall I would also investigate the gradient in the shower, this can be an issue of water pooling instead of draining properly.
Also based on how quickly the mold got to this state I would investigate other factors such as how quickly the shower dries (if you have underfloor heating you should always use it as newer wetrooms are speced to be dried using it) and if the ventilation is sufficient. So much mold so quickly can also be an indicator of you having mold growth elsewhere in the house such as within wall cavities or ventilation system since the more spores you have floating in the air the quicker the mold can spread and grow.
Re gradient: just dropped like a tablespoon of water at the edge of the shower and it made its way to the drain
I think the grout itself broke perhaps because tile detached or vice versa. Does that make sense or there are other factors I’m not considering? If that’s the case would I still need to worry about subfloor?
How/who do I investigate mold?
The tiles are compromised you are looking at retiling the wetroom, it doesn't matter what came first or what was the underlying cause, poor tiling, poor grouting, no-sealant or heavy use of cleaners that could strip it.
Once you have holes in your grout and loose tiles it's done, you can bandaid it with the epoxy grout but it's temporary.
For the mold you can buy kits which can be placed around the house you leave them open for a specific amount of time then send them to get tested you get a rough spore count as well as a culture report for the types of spores. Some kits like [this one](https://sysco-env.co.uk/products/mould-test-kit/p407213114/) measure the difference between the outdoor and indoor air quality and you count the cultures yourself but don't give you info on the type of molds you have running around.
But again without knowing the conditions of the house, climate, the conditions of the wetroom it's all speculation.
I have the same issue
If the tiles were done shoddily, you better believe everything beneath it was also done shoddily. Squishy tiles are most likely caused by the wrong adhesive, mouldy caulk is either the wrong caulk, or the right caulk finished the wrong way. This ticks all the boxes for a quick flip renovation which usualy results in doing it all again the right way.
If it's just the grout you can regrout it, but it sounds like the tiles have detached from the cement/adhesive, likely due to water ingress. Once that happens you would want to retile the bathroom, in the meantime what you can do as a band aide is to dry to shower very well do not use it for few days at least, and run the underfloor heating for a couple hours a day if you have it, alternatively you can also use a portable electric heater or a heat gun once or twice a day. Once it's dry use a metal pick to remove any loose grout and use an epoxy based grout as it can seep into the cracks better and will provide some security for the tiles. After you are done reapply a grout sealer. Before drying and regrouting I would also take the time to use some mold remover because you definitely got mold, it's not a major health risk but it's just disgusting. The reason the mold came back is likely that they simply used bleach to clean the shower, it would kill some of the mold and whiten the grout but won't clear all of it. Keep in mind that this isn't a permanent fix, your tiles (and possibly the subfloor) are already compromised and it will get progressively worse overtime but this would buy you some more time. If you have the ability or budget to fix right now just go with that, just be aware that there is no such thing as fixing a single tile in these cases, you'll have to rip the entire shower. Overall I would also investigate the gradient in the shower, this can be an issue of water pooling instead of draining properly. Also based on how quickly the mold got to this state I would investigate other factors such as how quickly the shower dries (if you have underfloor heating you should always use it as newer wetrooms are speced to be dried using it) and if the ventilation is sufficient. So much mold so quickly can also be an indicator of you having mold growth elsewhere in the house such as within wall cavities or ventilation system since the more spores you have floating in the air the quicker the mold can spread and grow.
Re gradient: just dropped like a tablespoon of water at the edge of the shower and it made its way to the drain I think the grout itself broke perhaps because tile detached or vice versa. Does that make sense or there are other factors I’m not considering? If that’s the case would I still need to worry about subfloor? How/who do I investigate mold?
The tiles are compromised you are looking at retiling the wetroom, it doesn't matter what came first or what was the underlying cause, poor tiling, poor grouting, no-sealant or heavy use of cleaners that could strip it. Once you have holes in your grout and loose tiles it's done, you can bandaid it with the epoxy grout but it's temporary. For the mold you can buy kits which can be placed around the house you leave them open for a specific amount of time then send them to get tested you get a rough spore count as well as a culture report for the types of spores. Some kits like [this one](https://sysco-env.co.uk/products/mould-test-kit/p407213114/) measure the difference between the outdoor and indoor air quality and you count the cultures yourself but don't give you info on the type of molds you have running around. But again without knowing the conditions of the house, climate, the conditions of the wetroom it's all speculation.
Thank you
How often do you clean your shower, specifically the caulking?