Interesting. The curl didn't increment, nor reset, the netstat unaccepted connections -- still at 15.
I'm using a Ubiquity EdgeMax ER-12 router with most recent (v2.0.9-hotfix.4) firmware, with no firewall rules on the LAN side, the Mac connected directly via 1GB ethernet.
I'm also using Little Snitch on the Mac mini, which for MacOS 12 changed the way they monitor and block traffic, moving from KEXT to Apple's new NetworkExtension API, although I had the network filter turned entirely off until recently, and only used it to monitor. Also, it's always had "Allow any incoming connections " and "Allow any outgoing connections" rules for channels-dvr.
I'll update my script to include that netstat.