I think I figured out how to reproduce it really quick.
- Log in to Lemmy on Firefox
- Switch to a different app
- Switch back to Firefox
- Reload the page.
Poof! I'm out. I repeated the test multiple times to confirm it's consistent. Probably actually getting kicked out on step 2. It's like FF decides to expire the Lemmy cookies when the app loses focus.
Armed with that info, I retested two alternate front ends on programming.dev (a. & t.) and default front end on lemmy.world and default front end on programming.drv with Brave. I was previously getting logged out on lemmy.world, but not now (maybe they updated something? Or my test is still flawed.). So, with these steps I can only reproduce it on programming.dev with the default front end on Firefox.