Remote Watch Pass incorrectly triggered for local playback (app.plex.tv)

My first thought is that it’s likely due to DNS rebinding protection in your router’s DNS server. This will prevent *.plex.direct IP addresses from being resolved, which will cause the behavior you’re seeing (because it will fall back to trying a remote connection). To test this, set the DNS for the Wi-Fi connection on your iMac to a public DNS provider, such as 1.1.1.1 (CloudFlare) or 8.8.8.8 (Google), or both.