Just posting to confirm that it's not a broken installation and hence it's not just Icho who has the problem, as my replay for Genocide 13 of United also broke in the exp build but is fine in the stable. This therefore makes me wonder why does it break there, while it's fine for Neutrality 38 and Tension 32. What's the difference with the blocker placements?
I think it is special for right facing lemmings.
Turns out it's not. After further investigation, there is a culprit that separates Genocide 13 from the other two United levels I mentioned, and that is
the replay breaks in the cases where you assign the second blocker from under the entrance after you already assigned the first blocker to the lemming who walks for a bit after dropping out of the entrance. However, if you assign the blocker under the entrance first and then the next one as close as possible as NL allows, then no replay breakage there.
The difference is that in the former case the blockers are as close as possible resulting in the tiniest gap between the blockers possible, while in the latter the gap between the blockers is at least a pixel wider. It doesn't matter if they're facing right or not. I checked with left-facing blockers, and the replays also broke if the conditions in the former were met and didn't in the latter.
It's not just limited to under the entrance. Replays also broke if you assign the second blocker upon immediately landing after a drop after you already assigned the first blocker to a lemming walking for a bit. However, after watching your Neutrality 11 video, seems that it only matters if you assign blockers to the lemming facing the same direction as the first blocker, but the important thing is they're as close as possible that NL allows which results in the smallest gap possible between them.
In short, what matters is two things, the blockers are facing the same direction and are close as possible as NL allows, and the blocker closer to the entrance is assigned after the first one that's farther from the entrance, resulting in the smallest gap possible between the blockers and hence replays break. Replays are fine if you assign the first blocker under the entrance/immediately upon landing after a drop first and then another blocker farther from it and as close as possible to the first one, but here the gap between the blockers is a bit wider.
So, it seems that what's important is the pass-through blocker is the second blocker, not the first blocker. Replays also don't break if the blockers are facing in different directions, since they're farther from each other than if they're facing the same direction due to the gap between them being a bit wider.