I don't know anything about this stuff, but MiNT should behave exactly as TOS, especially for programs that are not MiNT aware. This looks like a bug in MiNT...
If someone could explain the intention of this, there could possibly be a workaround in XaAES. Until then you can just disable this feature in MiNT by #defining MM_INSERT 0 in keyboard.c, as I wrote before. On my MiNT the Bconin-returns are ok now.
There are some keyboard-issues in XaAES, but not insert-related ... -Helmut