There is a difference between Mp3tag having the focus on a certain field on the Tag Panel and having the focus on the Tag Panel itself.
The former also enables the scroll wheel to switch between entries in the drop-down list. This is standard behavior on Windows. It's deactivated once the user starts editing in one field (as described by @JJ_Johnson's suggestion in [F] Tag Panel problems with 'wheel mouse').
The latter simply scrolls the whole Tag Panel (in case a scroll bar is visible). Further experiments with @ohrenkino in a private conversation have shown that this is only possible, if the Windows 10 setting "Scroll inactive windows when I hover of them" is enabled.