Foxes sleep on top of slabs, even in broad daylight

The bug

Foxes fall asleep almost instantly when standing on top of slabs, even when the conditions for sleeping are not met.

How to reproduce

1. /fill ~ ~ ~ ~20 ~ ~20 stone
2. Fly somewhere else
3. /fill ~ ~ ~ ~20 ~ ~20 stone_slab
4. Spawn foxes on both platforms
→ The foxes on the slabs all fall asleep pretty quickly

Cause

This is caused by the is_underground filter being really naive