So... I could reproduce this bug 100% of the time, but I just ran Picard in debug mode, and it worked correctly this time without crashing.

In summary, after tagging and saving a bunch of FLACs, I clicked the Submit AcousticBrainz Features button, as I've done many times recently... And Picard crashed on one of the files. I checked, the file is not corrupt:

{{$ flac -t 11

flac 1.3.3
Copyright (C) 2000-2009 Josh Coalson, 2011-2016 Xiph.Org Foundation
[...]
Ground Patrol - Art As Catharsis 2021 Sampler - 11 - Migration -.flac: ok
}}
I saved the Apple Crash Report (attached). And I repeated this five times, and it's literally just dropping the (tagged) FLAC file onto Picard, setting one of the fields to "Use Original Values" (because I don't want the album artist modified), and clicking on the Submit AB Features button. After waiting for a minute or two, Picard closes and the Report Crash to Apple dialog box pops up.

So, in preparation for writing this, I also started Picard like such:
$ ./picard-run --debug
...and now it worked without error. So, I've attached that too...