Premature session cookie expiration?

I've noticed that, even when I choose the "Keep me logged in" option, I usually have to re-log in to Tildes at least once or twice per day. Under what circumstances might the session cookie expire on such a short time scale? I can see that the Max-Age is set to one year, so I'm not sure what might be causing that.

Has anyone else encountered this issue? It's entirely possible that some of my add-ons are interfering (although I don't see how), which is the reason I ask.

I have to pull up my Tildes password and my TOTP generator in each case, which is just enough pain to encourage me to navigate to another tab instead.