As far as I've played, this bug is on 1.18.0. Idk if this bug is specific to 1.18 or is also in previous versions, but anyways, this is what happened:

Loaded up a pre-existing world, which had already been updated from 1.17 to 1.18. Spawned in a staircase at y=-18, down to do some strip mining (which is normal, as I logged out here), but right after the world loaded up I took 1.5 hearts of damage (with full iron armour).

Definitely not caused by some mob, since I left with no mobs nearby in a lit up staircase. Could be to do with the world generating and me taking fall/suffocation damage, not entirely sure.

Just a footnote: The screenshot shows me with 0.5 hearts of damage, but this is because I healed up a heart before I took the screenshot.