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.
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Duplicate of MC-212146