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No problem.
I think you should be able to update/reinstall it manually though if the store listing is delayed. Chromium 95 is almost 8 months old, so it's kinda weird that you can't update it.

My Ubuntu was a bit behind, but I've updated it and installed the latest Brave. Keychain working fine now.

I did notice it's not prompting me for a password when I first click Keychain after opening Brave. Has that behaviour changed?

Yes, we have to handle this differently now that Chrome is forcing us to move to their new manifest v3 extensions.
You can still change the autolock behavior in the settings though