Hey Get_Rext, I found a way to make it work by making a small modification to libretro.cpp and recompiling the core. I don’t know if it’s the right change to make, but it seems to make both US and JP 4P games work correctly, you can see the change I made on the rdanbrook’s github: https://github.com/rdanbrook/nestopia/issues/235
You might also have to make sure your game’s hash values are set up correctly in NstDatabase.xml (the translated Riki Kunio/Nekketsu Kakutou Densetsu didn’t work until I added its hashes).
Hopefully this leads to a more robust/correct fix, but for the time being I’m pretty happy to have this game working correctly on the RetroPie now.