Basically what's been happening for a bit is that every now and then I will navigate to either the forum index or a subforum index and at random the page will load in and I have been logged out. Only not really, because if I click on a thread or other area on the forum it takes me there and I'm still logged in. But if I go back to where it said I was logged out it will once again show me as logged out, even on a hard refresh.
Basically it goes:
1. I am in an SE+ thread. I move to the SE+ subforum.
2. Subforum loads in, I am in logged out status. I click on forum index.
3. Forum index loads in, I am still logged in. I click on SE+ subforum link again.
4. Subforum loads in, I am in logged out status. Hard refresh, still in logged out status.
5. Indeterminate time later. I move to SE+ subforum.
6. Subforum loads in, I am still logged in. I click on forum index.
7. Forum index loads in, I am in logged out status. Hard refresh, still in logged out status.
8. Repeat with variation
It never sticks in one place, can affect any of the forum or subforum indexes, and it comes and goes as quickly as the other so it's hard to pin down or replicate on demand. I'm using Firefox as a browser on desktop and Samsung Internet on mobile, it occurs in both but far more on desktop.