Hiya Jamie,
I don't agree, if there's movement, you'd find grut cracking, maybe tiles even, as the adhesive is breaking up. That would mean the adhesive and the grout is wrong. Seems t me like the grout brushes out, if it's cracked and coming away it would come away in lines, but stuck together still, maybe even ith bits of tile stuck to it as it's got to be sticking to something somewhere. If it's brushing out, and the tiles are still sound, he couldhave used an adhesive for grout, as that's not a wearing surface, or grout has been applied like water, which would usually be pinted out by low grout joints that are discoloured even after you've washed up and cleaned off.
Thank's for joining in though 😀 I can see where you're coming from. Maybe just in case, you should 'sound' the tiles Leanne to make sure they're all fixed. maybe get some of the grout that's coming out and check that?