During branch cleanup, I'll click a branch name on the left sidebar and check to see if it still has an origin/branchname tag/pill on the latest commit for the branch. If it does, when I delete the branch I'll click the "Delete remote branch" option on the "Confirm Branch Deletion" window - but a lot of the time I'll get an error saying "error: remote-tracking branch 'origin/(branchname)' not found."
So, if the Fetch feature's "Prune tracking branches no longer present on remote(s)" is set then why is this happening?