Shot 35 shows what I believe to be the regular loaders. Would you concur?
Obviously yes, that screenshot shows the loader positioned to a LOWER cargo deck so, since that is lower than 3.40 meters, it's absolutely normal a lower loader would be chosen.
Shot 36 shows the two airstairs. The two regular loaders can be seen just below the aircraft fuselage in their assigned positions. Still no Hi-loader.
The two airstairs are there because you haven't disabled the 2nd passenger door. And the two regular loaders are there on the right side, which means you haven't placed any of them on the left. Were you expecting the HI loader would be assigned to the location used by the 2nd stairs, "just" because the main cargo "seems" to act like a 2nd door ?
I believe you haven't fully understood how the GSX editor works.
First, there are TWO editors, and you are using the SCENERY customization editor. This editor ONLY set the initial STARTING position of the VEHICLES. It doesn't configure the airplane doors.
You must use the other editor, the airplane configuration editor, which doesn't have those orange/blue pointers, instead it has 3d arrows showing lateral, longitudinal and vertical coordinates of the doors.
Since you haven't shown any screenshot of THIS editor, which is what counts, I can't say what you did wrong, so I can only guess. As explained in many other threads, GSX supports ONLY TWO cargo loaders IN TOTAL. You just can't have two loaders on the right, and an HI loader on the left.
Which means, you should decide which one of the two cargo doors on the right will be used on the left side, and then MOVE its door, and ROTATE 180 degrees, so the green arrow that indicates the door orientation is pointing OUTSIDE, on the the left side of the airplane. It's usually best to use the *forward* cargo to be on the left side.
Now, after you have the correct config for the AIRPLANE, you might want or not use the SCENERY configuration editor, but it's not really necessary for the HI loader to appear (unless you configured the scenery to *force* the loader type on that parking spot to be a "BELT" type ), the HI loader will appear if the door's height is higher than 3.40 meters, the starting position defined in the SCENERY editor ONLY affects the PATH the loader will take to reach the door, not the loader type.