Sounds like you have cookies disabled for that site.I also do not allow my data and history to persist after I close my browser. So there's definitely more going on than just the auto-login. I just assumed that was it.
Regardless, I do have to login each time I want to view the on3 free forum.
FWIW, I use Bitwarden to auto login/save all my stuff.. I have it set up locally so I control ... everything and it's encrypted
For those that don't wan to go through that... their free version is pretty good and I used that for years.. none of your data is stored per se.. and I've never had an issue with getting info stolen or used elsewhere.
There is your last cyber-security-ish type tip for this forum