It is a known result of a fight finishing too quickly. If you're gonna use a bunch of 2* in your deck to run through matches faster, then you need to make those matches last a couple seconds longer.
yes, an initial 1-hit KO right at beginning will crash your game in BG.
Either put higher star champs as the throwaway champs in your deck, and/or hold BLOCK for first second (first hit by opponent), then you can leave it alone after that.
2* might even get KO by 1 hit even if blocking, so use 3*/4*. But 4*r1 can even get KO (without blocking) by initial hit since attackers are higher rank that year ago, so even those I do Block at first.