Take a look at the following gif:
(https://im4.ezgif.com/tmp/ezgif-4-ed47971635.gif)
And tell me, if a lemming goes up a slope and if there's a blocker on top of that slope, should the lemming turn around or nah?
I don't know if a topic about this has been made before but i'm making it now i guess
The middle part of the blocker area does not turn lemmings either way. This has several desired consequences, e.g. allowing to place blockers directly after spawning from the hatch, or easier freeing a crowd by building over the blocker, or not allowing turning around builders with blockers in such a way that other lemmings can no longer follow.
In your case, let's look at an ever more extreme example: Assume that the walking lemming is actually a climber climbing a wall with the blocker standing on top. Should the climber then turn as well? If yes, then fixing this would mean to remove the non-turning area in the middle of the blocker, which would be problematic in several ways. If not, then your example is basically just an edge-case of this that can be kept.
The Lemming is under the blockers turning area and can climb into the middle space of the blocker where no turn area is placed.
A blocker has a turning field at the left and the right arm, but no turning field in it's middle. This allows blocker placements under hatches and falls without the lems that fall down afterwards turning. Which is really conveniant in a ton of cases.
Also there had to be a rule how low the turning area actually is. On extreme slopes like this Lems can actually slip under.
Look into the true physics mode and you'll see it live with displayed blocker turn areas!
Back in the days we decided that we mostly prefer the lighter blocker behavior --- a no-turning gap in the middle as you see here, as it's more convenient in a lot of cases.
Stepping up steep is the only case where it can be slightly annoying sometimes when you want lems to turn, but even this behavior can be/ is used to make interesting levels around it
I still post this even Nepster was a min earlier :P