Broken font loading and weird behavior on Mint 21 - aegisub-procles

Hello. I made an unofficial snap for Aegisub, and it was a good experience, really. But recently I got a bug report from a Linux Mint 21 user who was having problems working with files on external drives, even with removable-media connection made. Opening files with Nemo was doing it in read only, and the file picker doesn’t show the external drive in the left pane, but manually opening the path in /media/ works. Also, the app has issues loading fonts. Launching the snap with FC_DEBUG=5 doesn’t show anything of note, no fonts from ~/.fonts/ or ~/.local/share/fonts/ were available. There’s no such issue on regular Ubuntu or any flavor, only on Mint, it seems. Is there any way to fix this?

UPD. Elementary has this issue too, and most of other snap software work with fonts just fine.