Reporting a bug here on retroachievements gui:
1 - Hardcore mode is set to ENABLED on GUI
2 - Does not reflect on ~/.config/retroarch/retroarch.cfg
# grep hardcore retroarch.cfg
cheevos_hardcore_mode_enable = "false"
3 - Manually edit that value to true through ssh, no libretro core overrides, launch game and right before starting the game, cheevos_hardcore_mode_enable is set to false again.
Turning on and of does not seem to reflect on hardcore mode on gui, which under the hood is always configured as false…