Okay so it looks like there was a larger playback issue with Plex for MacOS (and Windows) in v1.5.0 that was fixed in v 1.15.1.1358-43550df5
Installing v 1.15.1.1358-43550df5 restored the LiveTV/DVR playback on the Plex for MacOS app for my existing HDHomeRun Prime tuner, plus I now have access to the new LiveTV from Plex!
Thanks to the entire team.
