Some sources seem to get stuck at 1 unclassified image

[StephenChan]

This isn't giving us spacer errors, but it's leaving red rows in Backend Overview. See screenshot below - the rows where "# ver rbts" is at least 1, and "# uncl." is also at least 1. I can't think of how this might've happened.

[StephenChan]

My remark in PR #519 was misleading. It's cool to have "Unclassified, without features" at 0, but what this issue actually wants is "Unclassified, with features" at 0. That would involve separating out sources that don't have a classifier yet.