The bug

The bogged skeleton variant can spawn underground. This is different from the behaviour of other biome-dependent hostile mob variants, such as strays and husks.

Steps to reproduce

1. Go to a mangrove swamp
2. Wait a moment
3. Run the command /effect give @e[type=bogged] glowing
4. See the bogged spawn underground

Additional information

This may be intentional as there is little open surface in the swamps compared to deserts and frozen plains, however in that case their rates could be drastically increased in mangrove swamps to compensate.