I’d (personally) be interested in knowing if the sore is a recent/new issue on a horse you’ve had for a long time, and if there has been any change in saddle, saddle pad, weight/condition of the horse, or other change(s) that preceded the sore. Or was the sore just out of the blue, all of a sudden kind of thing and nothing is different? That might help eliminate/prove a cause. Like if you just bought a new/used saddle, and this just started happening, then I’d say yep, I think its the saddle. But if you’ve not changed a thing and the horse is the same, then that would be more of a head scratcher.
Silly question - have you washed the saddle pad you last used when the sore opened back up? The reason I ask, is that if you look at the underneath side of the pad, can you see where the sore was rubbing against the pad (excess hair, blood, scab pieces, etc. visible)? That may well give you a better idea of what/where the sore is coming from.
Even just at a walk the saddle will shift a tiny bit, more so if you’re jumping or sitting the trot/canter a lot. Certain saddle pads or blankets can exacerbate that movement (especially if the horse has very shiny/slick coat). As stated in others posts, pulling the pad/blanket firmly into the pommel/gullet can help - but as we all know, saddle pads don’t always stay where they are supposed/intended to stay.
There may be a simple solution like changing the type of pad (try 100% cotton instead of synthetic for instance), the shape of pad (square instead of “fitted”), changing the billets used for your girth (like if you use billets 1 & 2, try using 1 & 3 instead), try a good breastplate (for certain body types - horse OR rider - this can help the saddle from sliding) - it’s not always one thing or another, it might be a combination of things… and, yes that might involve saddle fit too. But it would be hard to say for sure until I knew the answers to my fist paragraph.