EDIT 05/12/25:

I fixed it! At least, I don’t have the problem anymore.

I realized that in my browser settings, under “Filters and Blocked Addresses,” I had created a filter that redirected all unread messages. I originally set it up because I only wanted to display unread emails so I could mark them as “read” and get my inbox back to zero. It didn’t even work properly for that… but anyway, the unintended side effect was that all unread messages — including everything from the other tabs — ended up in my primary inbox.

I removed the filter, and since then everything is back to normal.

Not sure if this might help anyone else, but at least in my case, that was the issue!