Observer with string triggered by entity on every login

I have a villager trading panel on a vanilla multiplayer server running 20w51a. There is a piece of string with an observer behind it in every cell so that when a villager drops into the cell the trapdoor shuts above it. However, when I disconnect and login to the world again it seems to trigger this on every login. It seems to be counting the reloading of the villager entity as triggering the string.