I've been tearing my hair out over this, but I eventually found this issue on github:

https://github.com/Chocobozzz/PeerTube/issues/4778

It explains why using nginx to multi-stream to peertube doesn't work, but what should I use instead?

I have purchased access to a VPS that I'd prefer to use, but I'd also be open to multi-streaming directly from my desktop. I am running Linux, which seems to make all the OBS plugins more difficult or impossible to get working.