I know this thread is quite old now but i would say this has the hallmarks of the tiles not being back buttered when they were fixed onto the floor. The tiler should have spread the adhesive onto the floor with the correct trowel depth and also back buttered every tile to ensure 100 percent contact. UFH has nothing to do with this otherwise the tiles would have decoupled or be hollow all over the floor area and not just in the corners of the tiles.