Butler sort by gray label not working as expected

[Ciarán Mac Lochlainn]

I've set up a rule in Butler which isn't working as I thought it should:

when a card is added into the list "QA", sort the list by label "Checklist" ascending

Checklist is a gray label, and there's only one card in the QA list with that label. I want this card to always be at the top, and I thought sorting by the unique label would achieve this. When I add a card to the QA list, the checklist card moves to the top and then moves again, usually to just above the new card. Sometimes it keeps moving back and forth repeatedly. There aren't any other rules on this list that could be triggering though.

Is there anything I can tweak to get this right?

Or (even better) is there a pin feature in the pipeline by any chance?

[Ciarán Mac Lochlainn]

Note I get similar behaviour if I sort descending.