Damage on spawn

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.