If yours is related to NOTICE: Unexplained Data Loss Issue then you’re right, they haven’t found a cause or solution yet. If you didn’t go through that topic, read through the initial post and provide as much info on that topic as you can to what they ask on information to gather.
Correct. They are encrypted using your OS keyring. If that file exists but passwords aren’t showing in Brave, then it makes it seem like your OS credentials might have changed, such as password and/or username. If not, something went screwy and it’s seeing it as if that information is not correct. The OS login (your user account, name, and password) are what controls the encryption.