Sometimes unenrolling from the macOS beta feed does not work properly.
You can fix this by enrolling / unenrolling through Terminal
:
sudo /System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed
sudo /System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil unenroll