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.
I had the same problem with loose tiles in my kitchen, There is a product I used called FixaFloor which is specifically for this problem. You drill holes in the grout around the tile then fill the void with the special Fix a Floor bonding solution which solidifies over 24 hours. once done you just put a little bit of grout into the holes you drilled and no one will ever see that you did anything to your floor!! It solved my problem and now my Kitchen tiles don't move any more! : ) Have a look at the videos on http://www.fixafloor.co.uk they show you how this works