This is a normal behavior - because:
During the authentication against any 3rd party you cannot set the tick “remember me” (even if it doesn’t matter). This means you’re not authenticated with a permanent session.
If your account then is inactive for “too long”, the session will be invalidated automatically - well before the actual limit you set. The same would happen if you close and restart your browser. I would have to dig around in the code to say the exact limit. If I’d had to guess it would be something in between 4 to 12 hours.
Usually well enough for this kind of working use case.