I didn't replace the star icons at all, because of their unique color changes (hover + active and hover.) So the star icons are still font icons.

This might explain why we are seeing this with the star icon, but not entirely why it happens at all. When it happened in beta, I believe there was a problem loading the wikiglyph resources. Can this be the case here?

Yep, I don't think it was caused by the conversion of the other icons. However, I never saw this in the past either (but I think Catrope did, before the other icons were changed).

It could definitely be caused by a load failure, but it's strange that it still shows something. It doesn't happen consistently, hence why I think it might be a race condition.

I've seen those a few times. They used to consistently show up if we had javascript disabled T85254: Flow's non-javascript menu icons are almost all wrong, but that seems to be fixed now (confirmed in opera and chromium with JS manually disabled). (I'm not sure if I should mark that as resolved or not, until we know why it's fixed?)
They also showed up during the beta cluster problems a few weeks ago. (IIRC, it wasn't loading bits.wm.o or javascript or something, for a day?)

It currently renders fine with no-JS. It might be an intermittent issue, but no-JS alone doesn't cause it.

Has anyone ever seen this again in the last 30 months?