Tiling over gap between wall and ceiling

We’re in the middle of a bathroom reno. We’ll be tiling from floor to ceiling. Our old bathroom had cornices but our new bathroom will have square set corners. A new ceiling was put in but we kept the old walls and now there’s a 40-50mm gap between the ceiling and the walls. Our tiler said he’ll just tile over the gap so the top 40-50mm of the tiles closest to the ceiling won’t be glued to anything. He said that should be ok because our tiles are big (600x600) and there’s no need to fill the gap. Is that safe or should we be getting him to fill the gap before tiling?

Best practice is a internal metal angle to join your wall with the cieling and tile on top. My only concern would be movement of house. I would imagine it will crack the grout.