This has been bugging me for years, most specifically with This American Life and To The Best of Our Knowledge.
The tag that's causing them to be separated is not available via iTunes -- you need an extended tag editor. On the Mac, I used ID3 Editor.
Instructions specifically for ID3 editor:
• Open a "good" podcast file (you can drag it straight from iTunes onto the ID3 Editor icon in the dock to open).
• Click on the Podcast tab. Look for the field called Feed (It may have a different name if you're using different tag editing software). Copy the Feed field contents.
• Open your "bad" podcast files, paste/replace the "good" Feed info into the Feed field (they should be different - mine were) and click Update.
• Last important step is that you have to delete the "bad" files from iTunes and reimport them. If you right click or control click the file in iTunes before deleting, and select "Show in Finder," the enclosing folder will open. If you sort by date modified, all the podcast you just updated should be at the top, if they're in the same folder. Then you can delete all the bad ones, and drag them back into iTunes. Whey they're re-imported, they should now be grouped with the "good" podcast.
I have been having this problem and while I don't use the rating quite the way you do, I get annoyed when new episodes are automatically assigned a rating.
I found this solution tonight for iTunes 12.5.1. I selected the podcast which was getting auto assigned (2 stars in my case). It's a very frequently updated podcast as episodes are short. I set View->View As->List. In the upper right corner there is a circle with three dots across the middle. Click on that and the rating option appears. I set the default back to no rating and all the hollow star auto ratings went away, even for episodes I was finished listening to.
Best Answer
Found a tedious solution on the Internet: Add all downloaded episodes to the new iTunes by copying the iTunes library and then select from the File menu "Add files to library". Go into each podcast's folder and select all files to add them to iTunes. Then iTunes will create the podcast with all these episodes, but you then have to subscribe to it again using the context menu on each podcast, so that it will be updated by iTunes.