Should be fixed in commit e25c4bb.
Basically, it arose because Nepster's correction for lemmings passing through one-way fields in a manner like this, worked by cancelling out a movement that happens unconditionally for a walker. However, if the walker turns around specifically due to terrain, then this movement already gets cancelled out in the walker's update code. The double-cancel thus allowed for the result seen here.
Also - nothing is special about steel here; this bug occurred with non-steel terrain too.