Tiler back butters the tiles and puts adhesive down adhesive too. On bal bag it says ccan be upto 12mm for localised areas. But agree would be difficult to fil voids now but will question tiler.
If he serrated the floor I would expect to see the serration marks beyond the tiles, there is one or 2 where this is not pressent and that's what initially drew my attention. + the shadow under the tile + the rather poor screeding. Jas, don't think I'm being piccy, I would have concerns about that floor if I where you. Time will tell how good the installation is mate but if I where you, I'd be lifting one of those tiles for further inspection to see!