It is supposed to at more stamina you hit harder. I don't get neither people running and spam heavy punch knocks you out +5 sec. 🙄 (You know the strength of knock out by the intensity of the slo-mo effect) If I do the same, then when grab I'm out of stamina. 🤷🏻♂️
Try other strategies like keep distance, and take an eye on stamina... Aim and dropkick, then spam normal punch, sometimes hits harder that you think.
I'm not sure if button spamming helps you to get up, but since you can play dead, I try to roll away ASAP.
This is Gang Beasts 2023, for sure is much better. It has more dynamic maps and Team modes are awesome, not just a Melee game. Have Fun! 🎉