edco can you share the output of your php flarum info ?
Were you able to check if the cookie are deleted, or if the sessions just expire ? What's the expiration date on the cookies ? Do they get removed, or are they still there with the same value while the session expires ? Can you maybe share a screenshot of your cookie tab in the browser console ? (I think both Firefox and Chrome have a way to display all cookies for the current site)
Maybe there's some cross-origin shenanigans going on ? Are redirects set up to your canonical domain ?
