Can be seen at around 51:50 in Ryemanni's video here: https://www.youtube.com/watch?v=e1Zl9qzrYeI
Basically, the lemming should not turn around. His first course of action if suitable should be to attempt to ascend the wall (if either it's only one slab high, or if he is a climber), otherwise, he should turn in accordance with the block's direction. He should only turn around in this case if the wall is not a deflector and not ascendable.
Related to this, a miner who is also a climber and hits a wrong-one-way-wall (maybe also steel?) turns around instead of climbing it.
Partially fixed in commit 7f0e759. Specifically, the turn-around is fixed, but the climber part is not.
And the rest is fixed in ce4cf77.