The only variable in your video is when you start holding block as KL. My guess is this changes the blocking animation. As KL breathes, his hurtbox shrinks and expands. When it expands, your knee hits him earlier in the animation, which affects how the next block animation plays out and the gap between the knee and QB. This affects KL's hurtbox again during the blocked QB. The later the QB hits them, the more advantage she has. That's my hypothesis at least.
I slowed down the video to .25% in youtube and watched the animation play out. Watch KL's left hand right before he gets hit by the QB. Sometimes it is outstretched towards Jacqui, sometimes it is closer to his own body (earlier in the animation). When it is outstretched all the way by Jacqui's belt (later in the animation), he gets hit by the next knee.
