When you use large format tiles, you must have flat walls. It may be that your wall needed to be rebuilt so that it is flat. Or maybe the tile setter just did not know what he was doing. I can see that he did not apply the mortar properly because of the gaps. 90%of the back surface of the tile should be covered, applied with a notched trowel. The wall is also covered with mortar with a notched trowel and then the tile is pressed into the mortar. The two surfaces of mortar are stuck together and it would take an effort to pull the tile off. Yours has gaps that allow you to peek down behind the tile. Not right.
If the wall was not straight, then you should have paid the workers to shim out the studs to get them even, then installed the wall surface. If you plan to never get those walls wet, it might be okay to put tile on drywall (I am no expert, cannot say for sure). But, if there is also tile over drywall in your shower, it must be torn out and redone. Even using basic cement board alone for walls in a shower is wrong. There are several methods for building a shower, but all of them require a step to waterproof the shower. Waterproofing can be a sheet of plastic properly installed behind cement board, or a membrane applied according to manufacturer’s specifications, or a surface treatment on top of the cement board like Red Guard, applied to the right thickness and using the right tape over the seams and screw heads, or it can be any one of many products applied correctly. The reason I bring up shower waterproofing is that tile people who do rotten jobs will often skip the waterproofing or do it incorrectly. If the guy can’t back butter a tile, can he do waterproofing correctly?
Q