From my experience if grout is coming out there's only one reason, movement or loose tiles. They may seem sound on first inspection. Put a glass of water on the floor filled right up and walk about on the floor see what the water does that should give you an indication of movement
Haven’t contacted tiler as wanted some advice before speak to him... tiles don’t appear to be loose, just crumbling grout which just hoovers out. surely a good tiler would identify If floor didn’t have suitable base and should have Advised remedy before starting
Hope he screwed all floor boards down where there was squeeking or movement,the subfloor the important one ,ply,or hardie ,then ditra to cover all angles
