Hi this problem happens more often than I desire and it's extremely aggravating. Because I will use the shift key to highlight multiple songs to drag them or delete them. And what ends of happening is the shift key locks up like I am holding it, and that causes my mouse to behave differently as well where it acts like I am holding the shift key. Whenever I am unbable to do that function it feels like it is out of sheer luck than me understanding what I am doing. I wish the program didn't do stupid things like that.
Do you have any proof that this is MP3tag (except that it also happens in MP3tag)?
The keyboard is managed by the OS - so which extensions do you have installed that intercept keyboard input?
Together with your other post about keyboard problems, I would check the local environment first, including the keyboard hardware.
That sounds like the behavior of "Sticky Keys" which is found in Win 10, Settings, Ease of Access, Keyboard. You might want to turn this accessibility feature OFF if you are activating it by mistake.