I think I've found the issue. There are two connections on the button as well as ground. I've switched few of the buttons over to the second connection and they all now function as expected. The ones I haven't changed still result in continuous firing.
I'll continue to rewire and report back if the results are good.
@batesman hi, i have been having the same problems with my 8bitdo. Did you ever manage to fix the problem? I just got my 8bitdo today and while i was playing super mario kart it just steered me into a wall on a few occasions, then started resetting the game or just going back to the game list menu. worked 100% ok playing golden axe 3 for megadrive so only noticed it in the snes emulator.