I’m glad I have email notifications on. I have found a solution to this since posting that thread! I realized some time after posting that the issue would occur not just with Krita, but with every app that used the default Qt file dialog, such as MusicBrainz Picard and Qt Creator.
I found, quite literally a few months later, an /r/ArchLinux thread in which the OP had the same issue, and they noticed the presence of file ~/.config/QtProject.conf, which had somehow gotten filled with garbage data, reaching 750MB. I checked and realized I had the same exact thing going on. Deleting the file immediately fixed the issue.
Deleting the file has caused no other issues to my system. I have no idea what it’s actually useful for, but Qt-based stuff does seem to regenerate it. I’m not really able to tell if it’s consistently ballooning in size or not but it might be worth keeping an eye on it afterwards if this fixes your issue and you do’nt want it to happen again.
Hope this helps!