I don't think that was what I was seeing. I had some that were not carbonated, and those didn't have large air bubbles in them. It looked to me like the inside of the spheres were carbonated, and the bubbles in them were building up at the top of the sphere and not escaping.
I made them, and they looked pretty good before carbonation. When I carbonated them, they got large air bubbles in them that made them look pretty odd. I tried serving them in a glass with some of the lime/sugar water plus a little extra rum, which was OK, but pretty odd. I also did have some trouble with them sticking to each other when setting in the bath, which was solved by gelling fewer of them at a time.