Mobs falling through world in growthplane after the Dec 15, 2019 patch
 

Rumbleroot clipped through flat ground near location -2034, +500, -55 during a 2019.12.17 PoG clear.

2019 12 17 Rumbleroot drops through world

If you look carefully it seems he may drop through the world right as he crosses a boundary where the sides of two ground tiles meet.