I put an empty folder in the Trash using the latest version of Thunderbird. It was named 2020 and had been created when I accidentally archived a message using their keyboard shortcut. Last night, I emptied the Trash (within RB 7), and the few messages that had been in there were gone, but the folder remained. It was clearly a subfolder of Trash, as the disclosure triangle was visible. I then realized that I had to click the three dots next to the folder itself and delete it that way. The folder disappeared from RB7. It’s not exactly intuitive.
As a side effect of that, however, folders removed using RB7 still appear within the folder hierarchy in Thunderbird, but they are grayed out.
I just ran another test, creating and deleting another folder entirely within RB7. When I deleted it, it vanished as expected from RB7, but it became another phantom folder in Thunderbird.
What’s causing this, and is there a way to make them as “gone” as RB7 thinks they are?