Is there a local saddle repair shop around to ask this question? Seems to me the saddle should stay flat on horse’s back when lunging. Just can’t remember what the back end rising indicates.
This is PRESUMING horse is on a long lunge line, in a round pen making BIG 60ft circles! If horse is on a short line, using normal 25ft lunge line with you standing still in the middle, holding 4-5 feet of line, horse body may be so bent that saddle can’t stay down. Curved spine does not allow saddle enough back length to actually stay under the saddle.
Ridden, the saddle stays down on the back because of your weight, probably less spinal bending going on than when lunging small circles.