Seen this thread for a while now, I wish I could help. I suppose one way you could find out is to just experiment and see if it's true.
Check if they respawn after 40 minutes of in-game time, and check again if they do when you're offline (check the other trees too).
If this never happened before, and it's an one off thing, then maybe it was a bug which you can report with the in-game bug reporter. Otherwise perhaps it's intentional and it was updated/changed recently.