Thank you for the reply!
Saving the fsm, deleting then pasting back the fsm worked.
This is a pretty annoying thing, because it happened again with an another prefab, and it's impossible to test when you have 500+ prefab items all over a 16x16km map, so I can only fix it when a player reports the bug. Then I fix it, update the game on Steam and all of a sudden another item is acting up and has the same variable replace problem.