It looks like there is a thin layer of air in the back where the grout didn't make complete contact with the glass edge toward the back, so you see this. It's like shoving cookie dough into a tiny fissure, not every part of the fissure is going to get stuck up against the dough. It would take days to hand shove each crack full to the point of full contact, if it could even be achieved. From the surface, you can't tell if the back of the joint is fully contacted, as you grout, and your joints are full, so your tile setter did a good job, and I would not try to suggest otherwise. The tile job looks very nice!