We've had a problem updating from the mirrored partner repo for quite a while, so I finally took the time to figure it out.
It *looks* like the problem is in the apt-mirror app itself. As described here apt-mirror wants to create an InRelease file. Not sure why it wants to create one locally when there isn't one upstream, but there you go.
As the post describes, our repo contained an InRelease file that was not on the Canonical site, and our local machines were using it instead of the updated Release file. Oddly, all other repos contain the InRelease file - I have no idea why this one doesn't, or why it suddenly created one.
So, perhaps this is less of a solution than a band-aid.
Anyway, the post recommends deleting the InRelease file from the $repo/dists/$dist-name directory and commenting out the two lines in the /usr/bin/apt-mirror script that look like this:
add_url_to_download( $url . "InRelease" );
Run the apt-mirror update and the file should not be re-created. Re-run apt-get update on your local machine and you shouldn't have the hashsum mismatch.