EPG not showing correct shows in CST with HDHomerun Premium Channels and OTA

Correct. Plex does not support this configuration.

You can vote for implementation of supporting this specifically:

And/or vote for EPG support for multiple tuners:

Another route is to set up a process that creates the guide data for you (XMLTV). Here are the posts that might cover that (found via search):
https://forums.plex.tv/search?q=hdhomerun%20premium%20xmltv