IME, most horses don’t just “try their luck” at bucking you off. Even the ones that might have appeared to do so because of the “randomness” of it turned out to have another issue. One that sounds quite a lot like you describe turned out to have terrible arthritis in the withers. That occasionally came out with flatwork but jumping was much worse. Or she was perfect. You’d never know, but when she bucked it was VERY hard to sit.
Other examples have been with horses with sore stifles, sore hocks. Sometimes this comes up as also lead change issues (as well as anticipation of the change), which you don’t have, so I’d put them lower on the list.
The most common IME for your issue is front feet. Spine (including neck and pelvis) would be secondary but less likely if he really is that good in flatwork. Now, there are plenty of reasons this problem could go away temporarily. More comfortable footing, could be as minor as how much moisture you have and how the ring was dragged that day. How high the jumps are–might be fine when they are lower. Where the horse is in the shoeing cycle and how the horse was trimmed that cycle. I had a horse who needed a lot of foot. He would not act foot sore until you jumped a big oxer then hold on! Left him a bit taller/longer and no problems. Some horses might need pads/pour ins. You may have very low grade, chronic subclinical laminitis that flares up from time to time. Knew a horse like this who was “randomly” very dangerous, especially jumping. Until he got a full blown case of laminitis that he didn’t recover from. Radiographs showed he had chronic inflammation in the foot for a long time. You may also have coffin joint arthritis issues. Mild navicular issues. So many things can be going on in the front feet that could potentially ebb and flow. While bucking can turn into a habit, if this horse really has the temperament described, I think it’s got to be pain, and it’s highly likely to be in the front feet.
Perhaps putting him in boot camp makes him suck it up for a while, but it doesn’t mean there’s no pain. After all, it’s not like he immediately goes back to being perfect for a pro. Might just get lucky that the stars align for a while and there’s less pain.