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.
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.