Sounds like something repeated drained your 12 v battery. The reset (recharging) of the 12 v is limited to something like 12 times so the traction battery doesn't drop too low, but somehow it apparently did anyway. Quite disturbing. Even in that situation, I had thought the engine would run to provide for driving and recharging. Apparently not!
2018 Kia LX HEV Metal Stream with Advanced Tech