Will look at this later. I already tested to check I could reproduce it - and confirmed it happens with both directions of one-way wall, with identical behaviour (lemming falls through and does not turn around).
I also will experiment to see if there's any way to set up such a situation on steel. I'd think due to the slight differences in how steel checks work vs how one-way check works (specifically, that one-ways are not checked for when assigning the skill, but steel is) it wouldn't be possible, but I should test this anyway.
With the current pause on physics updates it'll probably be a while until any changes to fix it, but it should go without saying that people should avoid making levels that depend on it.