Andor's Trail

I completed the Fungi Panic quest a few days ago (excellent by the way - thanks) where...
Quest clearly completed and was in my completed quests list. Today I happened to... I had the [Quest updated: "Fungi panic"] message again. Is that right? Surely if the quest was still ongoing it should not have been listed as complete. I don't recall seeing a previously completed quest subsequently updated again, although I might be wrong

I believe this was done on purpose in order to hide that little Easter egg.

It is very rare to get a new quest step after the quest is completed, but as you found, it can happen. Sometimes we leave unexpected things lying around in the game. If you find them, it's a small plus, but they are never necessary.