Simply put, the software sources mirror settings for "Main" and "Base" were reset to their defaults after installing the Mint 21.0 --to-> 21.1 upgrade.
I had "Main" set to MetroCast Cablevision and "Base" set to http://mirror.genesisadaptive.com/ubuntu, yet both were reset to their defaults after performing the Mint 21.1 upgrade.
this seems like it would be normal behavior to me but i could be wrong. Mirrors should be in sync, but during the upgrade, making sure things are set to the "main official mirrors" seems logical to me
Simply put, the software sources mirror settings for "Main" and "Base" were reset to their defaults after installing the Mint 21.0 --to-> 21.1 upgrade.
I had "Main" set to MetroCast Cablevision and "Base" set to http://mirror.genesisadaptive.com/ubuntu, yet both were reset to their defaults after performing the Mint 21.1 upgrade.