have you been doing something different since that time? or have you started using a different machine since then?
back in the day, we were able to track down a situation where performing certain actions in the system could trigger the system to clear your remember me token. that was fixed and seemed to eliminate most reports of this kind of problem, but it’s possible that there might be some other trigger that has yet to be well described.
the sporadic reports since that fix are mostly collected in another thread: https://forum.inaturalist.org/t/not-remembering-me-when-i-log-in/2426. you could try reading through there to see if any of those situations seems to be similar to yours, and if so, please make a note of any similarities or patterns you see. ideally, we need to find a set of steps that anyone can use to reliably reproduce the problem. otherwise, it will likely be very difficult to do anything to address the issue(s).