I'll make this my last post until you let us know how the rectification work goes,
Tubbed adhesive is not suitable for tiles of that size, there also seems to be a serious problem with the grout and the coverage of the adhesive behind the tiles if the water is getting through and filtering down behind the tiles so quickly.
Its a shame because in the pictures it seems to be quite a nice job but there are some issues that need to be resolved, sealing the grout may stop the leak but wont rectify the fact that the adhesive isnt suitable and possibly not safe, and that water has been able to get behind their in the first place.
If it were me I would try and remove a
tile in the affected area, I would suggest that it will come off easily and the adhesive will still be wet behind it. In which case the only proper and permanent solution is to remove and refix the whole lot with an adhesive suitable for the purpose.
Of course this in only my opinion as a humble tiler from seeing the photo's and reading the facts.
Oli