Not necessarily. Because of how the topline moves and shoulders open in canter, MANY horses will react only in canter. If the saddle is pinching behind the shoulders or bridging, often it will have its biggest impact in canter. For example my coaches lovely dutch warmblood started bucking chronically in canter with the saddle on, but was fine in trot. Took him to a chiro who said it looks like his saddle is bridging on his back. Sure enough, it was, she brought a new saddle and he hasn't even thought about bucking.