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.
It is me who basically wrote the keyboard.c module, but it was so long ago that I don't remember such details. I mainly used the Atari Compendium as a reference, and own experiments to verify, if the reference is correct. It is possible that it is written there and I missed failed to verify that.
Anyways, if you're sure that it will improve the compatibility of the keyboard behaviour so that it will behave more like TOS, fix that. It is the intention of the module to emulate the TOS behaviour as closely as possible.