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
MC-212184
Glow Lichen generating in air in Mineshafts
Resolved
Duplicate
0
iNkoR_the_2nd
(Unassigned)
Unconfirmed
(Unassigned)
21w03a
Retrieved
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.