That is the Fastmail Junk mailbox.
Now it has sub mailboxes - one is Fastmail, other On My Mac BUT there were no sub boxes then and I couldn’t select it … that was the problem I described earlier. 
This screenshot is the top of the sidebar and has what I considered to be the Junk Fastmail box.
It looks like you formerly had the Fastmail account configured to use a Junk mailbox under On My Mac. But now it shows up here because you’ve selected a different mailbox as the special Junk one.