Xastir Interface Text Not Appearing

I've just installed Xastir on an Ubuntu Groovy box. When trying to use it, important parts of the interface text don't appear (per screenshot)

(Entries are definitely present, I can select different "Interface Types" per the visible selector, I just can't see what they are!)

I suspect (based on button text) that it's rendering white text on a white background.

This is a clean install of Ubuntu-packaged Xastir on a relatively vanilla Ubuntu Groovy box (recently upgraded from Focal where the same issue occurred).

I've wiped ~/.xastir, this hasn't helped. I've looked in ~/.xastir/config/xastir.cnf and tried poking assorted *_FONT config values, which hasn't helped.

My suspicion is that this is some kind of system / X-based config issue, but I'm not familiar enough with the tooling to really debug it. I can't reproduce it in another application but I've not found anything that uses similar window rendering. It could be a Xastir issue, but it doesn't feel like it.

How could I debug this further please?