Hi,
sorry for getting back to you only now, but the Christmas period didn’t leave me much time to do extensive testing.
By now, however, I’ve tried quite a few things and can say that the issue always occurs the first time I encounter Fifi, regardless of whether I’m playing as Buster, Plucky, or Hampton. I also tested this on a GBA SP, where the same issue occurs.
I’ve attached two short videos demonstrating the problem:
I’m using a flash cartridge writer that allows me to write my own cartridges so the experience is as close to the original hardware as possible. I’ve never had any issues with it before:
https://shop.insidegadgets.com/product/gbxcart-rw/
For testing purposes, I also copied the ROM to a Supercard SD and tried it on the GBA SP as well. This time, the error did not occur but I experienced slowdowns and graphical glitches.