So I did some more digging, and I found that streaming from iTunes to an Apple TV or another iTunes client sets “NetworkClientActive” mode on the server and the Mac does not go to sleep during the stream. Plex streaming uses “PreventUserIdleSystemSleep”. iTunes keeps the Mac awake during stream, Plex server does not. Mounting a file server from the same mac also uses “NetworkClientActive” and also keeps the mac from sleeping.
It could be a bug with macOS 10.13.2 and PreventUserIdleSystemSleep. We’ll see what 10.13.3 does (should be out soon) and I will follow up again.
I’m surprised no other Mac users have reported this. I guess most people aren’t using a mac as a server, or are keeping their server from sleeping, or it has something to do with using older mac hardware as the server.