Npc disappear when knock out

Please be as descriptive as possible!

Game mode: Online | private server
Problem: Crash | Bug | Performance | Misc
Region: America
Free text
Multiple times when we have knocked out an npc they will fall through the ground. Especially happens in sandy areas that are not completely flat, but has also happened on stone ground as well. Makes capturing thralls a lot more work, and my luck is that is always seems to be the ones I really want lol.

Repro steps: