Sometimes Glow Lichens can spawn on air and not on blocks. 

This happens when two Glow Lichens are connected and one spawns on a block and the other spawns in the air, although this can definitely happen when they aren't connected but just actually floating in mid air.

Recently I found out that this bug can occur in caves and not just underground structures like mineshafts and strongholds. Unfortunately I was unable to get a screenshot of this bug occurring in caves and not in underground structures.

I have attached images.