Previously I do not remember having different stairs in the same operation on the ground.
Because many users COMPLAINED about always seeing the smallest stair with no people on their 737 so, we allowed the largest stairs there too, so now they both have a chance to appear, especially on the rear, which is higher than the front exit.
This happened several updates ago so, it's not really new.
But now I'm commenting on your previous post, in which there wasn't anything wrong. But now your last post show again the ladder, which is not supposed to appear. In any case, I think I found the problem, there was a ladder which were allowed just because the parking size was equal or less than 18 meters, and yours is exactly 18 meters so yet, that ladder had a chance to appear.
If you run the FSDT Live Update now, it should be fixed. That ladder couldn't reach higher than 2.2 meters so, I've added an extra constrain, so it will never appear unless the door's height is lower than that.