I've never seen MC change capitalization on anything unless I told it to. Surely never just from changing the genre.
The only thing I can think of that would cause MC to automatically change tags is if Auto-Import ran in the background and was set to upate for external changes (Tools->Options->Auto-Import Folders->Options), but that would still require that something else other than MC actually changed the tags.
It gets tricky when two tagging programs are thrown in together. It's hard to figure out which one is making which changes.
But until it gets figured out, you can go to Tools->Options->General->Import & Tagging and uncheck "Update tags when file info changes". That will prevent MC from physically writing to your tags. Then later when you're sure you want it to, just check that box again, select all your files, right click on them and choose Library Tools->Update Tags (from library).