Finally found the solution:

The default configuration of GPIONext is not optimal for those inputs.

What fixed it:

gpionext set combo_delay 0 # was 50

Hope it helps someone else !

@barbudreadmon thank you very much

@supercatfooz ahh ok sucks ah well can't want everything right heh
But it would be awesome tho if it worked

I remember playing a lot of Bomberman '94 on the PC Engine/TG16 with my girl friend back in the day! This version supported up to 5 players at once! I think there's been a Bomberman/Dyna Blaster game ported to just about every retro console, handheld and the arcade.
https://tiredoldhack.com/2017/02/26/the-complete-history-of-bomberman/