Confirmed in 18w21b.
Mobs do spawn in underground; they never spawn under the sky (the same for the End) in chunks created in 18w16a.
Confirmed in 18w20b.
How to reproduce:
Create a world in 18w16a with cheats on.
Teleport to somewhere away from the original region. (not sure if this is necessary)
Explore around the region a little bit(not sure if necessary)
Convert the world into 18w20b.
/set time night
No hostile mobs will spawn.
The bug may also frequently cause burning or temporary suffocation when entering Nether portal in multiplayer. I guess that the coordinate translation happens after switching dimension, and the suffocation/burning/falling happens between these two stages.
Well as far as I've played it looks fixed.
Thanks
Have the same experience on Fedora 20, Java 1.7.0_60 OpenJDK
Attached a screenshot.
Looks like fixed in 1.7.2
World save file attached.
Added reproducing method.
Still occurs to me in 18w22a.