Depending on the specifics of the saddle and the horse, some lift in the back can be just fine when the horse is moving round, and if the saddle is designed to be more of an upswept fit. It doesn’t necessarily mean it’s not supported in front or along the weight bearing area. It can help horses who are sensitive in the loin, have a shorter back, who aren’t at a phase in their fitness where they are really engaging their glutes as they should which would bring up the croup and lower back more.
But significant bounce is bad as is the even worse double bounce feeling when posting. That is an unstable saddle that likely has a small area taking all the force. I know the horse is leaving but you can try shims in the front or a thicker pad to see if it is lifting because the front is too wide or doesn’t have enough support when girthed up with a rider.
I have seen Prestige fit a friend’s dressage horse with major cantle lift on purpose…wasn’t so obvious with a rider but on the longe it was so far off the back. That’s too much.
