How to fix or re-install the broken packages resulting from updates installation?
This seems to be an issue with the python3-software-properties package or some of its configuration files, that might be providing inaccurate information to apt-get. I thus suggest you to purge this package:
sudo apt-get purge python3-software-properties
And then install muon:
sudo apt-get install muon
This later command will force a fresh install of python3-software-properties and any other packages muon may depend upon.
For more details on apt-get purge, I suggest this answer.