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.