That's because MM works with a database (a collection of previously scanned id3 tags). You could even delete all your songs and still see this tags in MM. Don't do it anyway. 
Mp3tag read the tags directly from the tracks every time you load them. So you see every change instantly.
You should update your Mp3tag to the newest version and turn off "Preserve file modification time when saving tags" in Mp3tag. This way Mp3tag changes the last modified date for your changed tracks. This should show MM to re-scan such files.
Please read this thread in the MM forum too.
Update 26.05.2025 for future readers:
You can find the option "Preserve file modification time when saving tags" in newer Versions of Mp3tag in the section:
File -> Options -> General