The option to have Muon Update Manager notify the user of updates does not seem to “stick”. if you open the app again the option is once again unchecked. To have it actually retrieve a list of updates you have to do a manual refresh.
Can you check the service as active in the systemsettings ? Or did you do that already ?
Yes, the settings in the applications settings dialog indeed do not change / stick between launches.
This has been how Muon has been for a while now.