OK, with further testing, this appears to be solely related to downloads, not uploads.
When logged onto no hubs, it doesn't happen.
Logged only onto a hub where sources for my downloads are, it immediately happens.
Logged only onto a hub where people are downloading from me, it doesn't happen.
After setting my entire download queue to "paused", logging back onto the hub where sources are, it no longer happens.
Now I know strict management of my download queue will control it, I don't even know if it's a bug at all. The only question I'm left with is why this never showed up for me in earlier versions.