Ok.That’s just weird. The problem sounds like the keymap not having the right number of entries. The only thing I see off the top of my head is that you have a missing \ on line 74 in your new code. That /might/ be the cause.
There’s nothing inherently special about the Fn keys. The only key that has some extra specialness is Prog – it’s also hooked up to the main controller chip, but that’s neither here nor there.
If fixing the \ doesn’t repair things, I’d recommend starting with the default Model01-Firmware and doing something really trivial, like swapping the A and S keys. If that works, then back it out and