It's a problem that people have when installing glass tiles. With regular tiles, all you see is the front of the tile; with glass tiles, you see all the way to the back: the slight imperfections, including any places where a bit of grout is missing, or where there is any chipping in a cut tile, or where the backing is inconsistent, or the tile back has ridges that aren't 100% filled with grout.

It's not "settling"; it's imperfections that commonly show up with this type of installation. With skill (and luck) this can be avoided, but it is always challenging to get this type of installation perfect. I've had glass tiles installed as a contrast border in a shower, and this single band was a challenge even for my otherwise highly competent tile contractor. After seeing how difficult it was, I resolved to avoid using glass tiles anywhere in the future. Unfortunately, if you are not aware of this potential issue, it's not one of those things that is obvious in advance (or that tilers usually warn about). Once installed, there is no fix short of ripping it out and redoing it, which also compromises the waterproofing membrane on the wall.

Wish I had better news for you, but that is the situation....