What model of the console are you using? I only have a PAL TG16 myself, so that's the only kind I can test with.
I really have zero documentation about how the cart works. The way I made the updated menu was by manually disassembling the original menu with the help of an emulator, and then adding my own code for listing multiple games and loading some additional graphics. But the actual loading should be exactly the same, so I'm assuming you have the same problems with the original menu(?)