I'm putting the saddle behind her shoulders. A couple times I put the saddle even further back than the shoulder to see what would happen - same thing.
I recently took out a saddle on trial. Granted, it was an English saddle, but I had the same issue of it sliding forward pretty far. I was not able to ride in it as we were plunged into a deep freeze in my area of the country and riding in -10 F with no arena...well, there is just not enough money to entice me to do that.
However, the saddle was used and something I did not want to let slip away if it fit him.
My boy has no saddle "pocket" to speak of and is made like a barrel in the shoulder area, but I see your horse has some definition....
So anyway..wondering where you are with your saddle search/fit and wondering if the uber short backs of our horses are part of the sliding issues.