Huge problem and sleepsless night ,opinions please

[Stef]

The 1st thing I notice is that the tiles haven't been back buttered,
Very little bond on back of tile.
Onto ply, even though it's been overboarded with ply I would still try & stick a backer board down.

 

[beanz]

How long before the underfloor heating was turned on? Lack of back buttering could also be a contender here...

 

[Rookery]

Just my opinion but I'd say:
1. Too much deflection in middle of floor.
2. Tiles not back skimmed.
3. UFH switched on too soon and not warmed & cooled gradually.
I wouldnt have tiled to ply either.

 

[radim]

two different materials expand in different way when heated (tile and adhesive) and this may lead to crack. So when tile debond from adhesive it is actually horizontal crack. IMO