There's a similar bug where the assigned level of npcs change to a different value for no apparent reason.
For example I spawned in npcs that have different default levels and put them all into the same encounter, then adjust the level of the encounter to whatever I need.
Say I spawned a level 1 npc, a level 5 npc and a level 10 npc, put them all into one encounter and adjust the level of the encounter to level 4.
Then I leave the scene, but when I come back to it later the level of the encounter and npcs is sometimes changed. Instead of still having the assigned level 4 they sometimes change to a seemingly random level, like instead of level 4 the encounter is now set to level 6 etc.
I couldn't figure out yet what is causing it, that's why I described it as random. Maybe possessing npc's might be the cause for this aswell?
