https://steamcommunity.com/app/892970/discussions/0/2950411088485696366/

One skeleton respawns outside cleared burial chambers, unless you have something that prevents spawning there.

They can even spawn very quickly: one time I had a workbench at a dungeon entrance, and like 10 seconds after I removed it (its purpose was to stop spawning while I was mining copper there) a skeleton appeared.
I guess there's a timer on it that my workbench was preventing from hitting zero.