I have a freshly installed OpenSUSE Leap 15.3 (plus some more software) on a laptop, with encryption, and I’ve set it up so I only need to enter the password twice. After I boot, it unmounts most volumes (apart from ones it fails to unmount because they’re in use, such as /var). I’ve also reported this at https://bugzilla.opensuse.org/show_bug.cgi?id=1190272, along with the journal from when it happens.
You have snapd installed, where it comes from? I do not see it in default repositories.
Do you observe the same issue if you disable (or remove) snapd?
I have snapd installed from the system:snappy OBS repo. Uninstalling it doesn’t help though.
Did some more experimentation, and I can trigger it by hibernating, resuming, and then rebooting.
I have done some changes to allow hibernation to work, specifically adding a dracut config file to make it load the resume module, adding the resume kernel parameter in /etc/default/grub, and adding force to the swap partition in /etc/crypttab so the initrd will be able to decrypt it.