Sometimes lava doesn't flow down

While exploring the nether, I found some lava defintiely did flow down, but came across one that didn't.

This is a 1.17 world, upgraded from 1.17.1, but I am pretty sure this is a 1.18 chunk. (newly generated)

This was upgraded to 21w44a from a fresh backup, so its only been upgraded from 1.17.1=> 21w44a

seed: 1823439624  (also in screenshot)