Normal, expected behavior
For security, an inactive session signs out automatically: after 45 minutes of inactivity normally, or after 2 days if you checked "Remember me" when logging in. Using the site at all, clicking, scrolling, typing, resets that timer, so this should only ever happen after a genuine period away from an open tab.
If it's happening much sooner than that
The most common cause is your browser clearing site data between visits, common in private/incognito windows by design, and also possible with aggressive tracker-blocking browser settings or extensions that treat all site storage the same way regardless of whether it's tracking-related.
Try a normal (non-private) browser window, or check whether an extension is set to clear cookies/site data automatically, if the issue disappears there, that confirms the cause.