There are two cracks, each running as a gently waving line through a line of tiles from ceiling to floor.

Installer suspects that the plywood he used may not have been of the quality he specified. He wonders if it has bowed. The cracks follows lines where two sheets of plywood meet.
The tiles at that point have become slightly concave. Hence the cracks. But he's not sure if this is the explanation or not. He can't see how the plywood could have bowed concavely - convexly seems more plausible. He used a flexible adhesive.

All we're doing at the moment is monitoring to see if any other cracks develop. But rectification is going to be tricky, and messy.

I would be so grateful for any input. The bathroom was finished approximately 2 months ago and has been in use for 6 weeks. The building is a very old one (100+ years) - so it has nothing to do with foundation settling.

Thank you!

You need a better installer. Plywood is not a suitable substrate for a tile wall in a bathroom. Cement board, wonderboard ro Hardiebacker should have been used. He has created a mess that can not be fixed. Make him take it all down. If you are unsure call 3 other licensed tile contractors and ask their opinion, not one would use plywood.

Thank you so much! What a headache - but I can see beyond a shadow of doubt that the installer needs to make good on this monstrous installation. Supposedly, he has installed 8 other stone-clad bathrooms. Heaven only knows what they look like! I appreciate your advice very much. Trish