I'm currently playing a world created in 18w16a and then converted to 18w20a, b and c.
I can't tell at which one of the 18w20 patches this started happening.
Basically, during the night, aggressive mobs only spawn underground.
I'm in a mine at y = 11, and mobs spawn here during the night.
As soon as it becomes day, zombies and skeletons start to burn like they would do overground.
Nether isn't affected.
I also noticed constant daylight in a little cave whit no torches that served as a mob spawner, and now it's completely useless.
Seems like a world conversion error.
Related issues
Comments


brand new 18w20c world -> normal mob spawning overground and normal burning at day
Another world created in 18w16a, converted to 18w20c -> bug confirmed, mobs only spawn underground and they burn during the day even if covered from daylight.
Should I try to convert a 1.12.2 world?

Yes, please.

Mh I'm sorry but it seems I'm not able to convert a 1.12.2 world to 18w20c.
the process suddenly stops using CPU and only loads some KB of RAM every now and then, the game is stuck at Loading (the process respods, windows doesn't tell me that it's not responding).

For the 1.12.2 to 18w20c issue, see MC-129374 : Install 64bit Java or increase stack.
Please report if the issue is persising if you play a new 18w20c world ?