Im on a realm and we have the same issue
This should be reopened, I'm having this issue in my realms world
confirmed in 13w21b. Walking south allows player (in any gamemode) and some mobs to pass through the "arm". Walking north will only allow player to pass through if they jump near the piston first.
We have better glass,smooth quarts, better logs, faithfull, invisible item frames, better glass,small totems and turning them off we still don't do anything. It's on a realm and the host is on pc and he has tried copying and redownloading but that doesn't work either also it's a market place map if that helps