I don't find today any filed bug about this in gnome-disks tracker: https://gitlab.gnome.org/GNOME/gnome-disk-utility/issues and I'm not finding the way to register a user account to file a new bug report there.
I've made some testing and bug seems only reproducible with LUKS2 formatted volumes. LUKS1 ones work perfectly, and gnome-disks by default uses LUKS1 when creating new encrypted volumes. I believe main problematic scenarios are: 1. When using gnome-disks to change passphrase for volumes created during OS installation (Buster's DebianInstaller formats in LUKS2, and no one of Cyril's proposals have been implemented, as expert install question or kernel line option) 2. When using gnome-disks to change passphrase for volumes created with other software, and user doesn't care about LUKS version. Easiest solutions seem to be now: A) Switching back DebianInstaller to use LUKS1 by default. B) Not including buggy gnome-disk-utility in repositories, until Gnome's bug is solved.