Brave forgets every 2-4 weeks most login data

OK. I think I understand what is going on.

Brave (and some other apps) will store passwords in a local ‘keyring’ which is usually managed by some app such as Seahorse, for example. Most Linux windowing systems will be set up such that, when you enter your password to unlock the system, that password also simultaneously unlocks the keyring.

If you use autologin, or if you log into the device using biometrics, that password is not available to unlock the keyring so the stored data likewise remain inaccessible.