I don't know if this is an Mp3tag issue, or something wrong in my system, but when I open Version 3.18 (including the betas) the main window opens behind all other open windows ("Z" in Z order). I'm running the latest Windows 11 version.
EDIT: This only happens when launching from the context menu.
It only worked briefly, then reverted to the previous behavior.
Here's the reply I received from Christian Ghisler, the author of Total Commander:
Sorry, I don't know what else you could do, except for contacting the author of Mp3tag. Total Commander launches all programs with option SW_SHOWNORMAL. But the context menu item is a shell extension created by mp3tag itself, so maybe it's calling the program with the wrong parameter SW_SHOWNA (show, but do not activate).
Someone here on the message board posted instructions for implementing an Mp3tag button on the vertical button bar in TC. It works great, and the program opens as it should. I'm fine using that instead of the context menu, so I'm just going to chalk up the context menu thing to some peculiarity on my machine. Thanks for your help, Florian.