It's possible it might have ridden up as he looked behind or when he turned back.

Those goggles look very tightly connected to the helmet and look how often Vader's helmet would stay put even though Prowse was turning one way or t'other.

If that is the case (and it's been a few months since I've seen ESB so I can't be sure) it really should stay as that would happen to Luke as much as happen to Mark.

Who is to say that he didn't pull it back into the correct position later while the Force/Camera/Audience wasn't looking?