Tested this again and it seems to have been fixed at some point by some Ubuntu (18.04) updates: my test user still had version 8 of the extension, and I could no longer reproduce the issue, neither before nor after updating the extension to release v9. My main user’s desktop now also appears unaffected.

(I was going to try the workaround reported by @ChrisLancs, but ended up not having to. My ”List type” is set to ”Disabled”.)