The GBA has a physical switch inside the cart slot to activate the GBC chipset (and deactivate the GBA chipset) when a GBC-sized cart is inserted, so no GBA flashcart should be able to run GBC roms natively, because... it's a GBA flashcart, and is GBA sized, and it can't physically reach the GBC switch in the cart slot.
Any GBC on a GBA flashcart should be running in a GBC-on-GBA emulator, regardless of the hardware's ability to run perfect GBC.
Any GBC on a GBA flashcart should be running in a GBC-on-GBA emulator, regardless of the hardware's ability to run perfect GBC.