Might also be a problem with other structures or elements, but I only observed this in a mineshaft corridor. The lichen generates in air, obviously unintendedly so.
Seed: 8488533685089907920
XYZ: 256 39 -358
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anoos7
Already reported this bug MC-212146
My case was in a stronghold.
I think what happens is the Glow lichens spawn in caves in the structures, so it stays floating, unless it gets block updated.