4-Player not working (1x player controls 2x different players!)

[Dochartaigh]

Trying 4-player Golden Axe: The Revenge of Death Adder (GA2.zip). I can't get all 4-players to work...

First time I tried it Player 1 character selection works fine. Player 2 selects both player 2 AND player 3 (controls seem to be mirrored). Player 3 (now player 4 since player 2 selected 2x characters at once...) works fine as well.

I'm using RetroPie, on a Raspberry Pi 4, running newest lr-mame2016 built from source. When I re-built from source again (took~40 minutes!), Player 1 works, Player 2 works Player 2 now (NOT 2 and 3 like before), but Player 3 & 4 are non-responsive. Went into settings double-check Player 3 and Player 3 magically starts working... still no-go for Player 4 in-game. Player 3 also ONLY lets me choose the Player 4 character... (this could have happened with the original built too FYI - I didn't go into the settings and mess around back then).

RetroPie says that lr-mame2016 uses the regular ""RetroArch control configuration"... meaning that if other 4-Player MAME/arcade/FinalBurn cores are working in 4-player (like FInalBugn, Mame2003, etc. ALL work for me for 4-player games), that lr-mame2016 should too, but it's not.

What can I try? Mame2016 is the only one which has the shadows working properly in this game (otherwise I would just use another version of Mame).