I play mostly Rick and was wandering the same thing. I could be completely wrong, but i think it may depend on which way the enemy is moving out of hitstun. For example if the enemy is holding towards Rick, I think that is when the combo drops.
Love how half the cast is too short for Rick's laser gun and how often side 2 misses for no reason, especially against Reindog debuff form. Like, he doesn't even need buffs, he needs FIXING lmao Also, seeing that Finn get humbled was amazing, he's gotta be the worst match-up with this character