So can you lift a tile in the dry area? This will tell you if the water has leaked into this area and probably warped the ply causing your loose tiles.
The grout lines are not your problem to be causing the leaks, it is what is going on under it, i think @impish is probably bang on and your tanking liquid has de bonded from the tray taking the tiles with it. You haven't mentioned adding a membrane this time which is the main advice being given so add that to your list. Remove the 12mm ply and replace with a backer board something like 12mm Jackoboard which is waterproof also. CTD sell it if you have one close and for the size of area you need you won't be spending much more than ply.
Paul is also correct that you will most likely need to remove the first row of tiles as all the taping/tanking seals will most likely be destroyed during removal so it would be best practise to do up to this row again.
You can do the mosaics no problem but yeah go for Epoxy, belt and braces this time!
The grout lines are not your problem to be causing the leaks, it is what is going on under it, i think @impish is probably bang on and your tanking liquid has de bonded from the tray taking the tiles with it. You haven't mentioned adding a membrane this time which is the main advice being given so add that to your list. Remove the 12mm ply and replace with a backer board something like 12mm Jackoboard which is waterproof also. CTD sell it if you have one close and for the size of area you need you won't be spending much more than ply.
Paul is also correct that you will most likely need to remove the first row of tiles as all the taping/tanking seals will most likely be destroyed during removal so it would be best practise to do up to this row again.
You can do the mosaics no problem but yeah go for Epoxy, belt and braces this time!