laiz I am using standalone username/password login. I created a new profile to make sure this was related to the browser itself and not some config mismatch between upgrades, if any.
charlyt1d it also happens in Github. Again, plain old user/pass with some 2FA, but that shouldn't make any difference IMHO. I cannot reproduce this with a custom mail server however.
I tried emptying the cache, but that didn't make any difference either. Deleting cookies is kind of pointless since this shouldn't happen at all in the first place, but in case you are wondering,
it keeps me logged in on Private mode. it sure logs me out (sorry, I didn't test this correctly the first time).
I am using Mojave, so maybe that could be the key to the puzzle.