The issue is actually worse than i thought, adding coins on the libretro port does something different from standalone on every neogeo games i tried :
- on standalone you add a coin, the game returns to title screen, the music keeps playing
- on libretro you add a coin, the game returns to title screen but there is a black screen in-between, and the music restarts from beginning
I think this behavior has always been there in the libretro port for neogeo games, it just never crossed my mind that it was different in standalone. And i think the half-sound bug in viewpoint is somehow related too.