Why do my baguettes split open along the seam in the oven?

Your instinct about the seam is right. I fought this exact thing for months and it turned out to be shaping, not the oven. When a loaf opens along the fold line it means the seam is the weakest point in the dough, so the pressure from oven spring finds it before it can lift the scores. Pressing with fingertips usually is not enough. You need to actually pinch it: after you fold the last flap over, use your thumb and the edge of your palm to press and roll the seam together down the whole length, almost like sealing a long envelope. It should look and feel like one continuous line of dough with no visible flap edge. Then rest it seam side UP on the cloth. That's the part most people skip. If the seam sits down against the floured couche, the flour keeps it from bonding as it proofs and it half unseals itself. Seam up keeps it closed and lets the smooth tensioned side hold the shape.

On the fan oven thing, I bake all my baguettes in a fan oven and it is workable, you just have to compensate. Mine is a small 60cm wall oven and I dropped the temp 15 degrees compared to what recipes said, because that moving air browns the crust way faster than static heat. I also steam twice: boiling water pan at the start, then a quick spray on the walls at the 3 minute mark. And score deeper than feels right, maybe half an inch at a shallow angle. Shallow scores just tear open into the flap thing you are describing even when the seam is fine.

One more thing that helped mine: stop the dough from sticking to the cloth along that seam edge. I was using way too much flour on the couche and it was drying the surface out where the seam sat, wich made cracks even worse. Now I use just a light dusting and keep the loaves covered with a towel while they proof. Also don't overproof, if they're past puffy and going slack the seam gives up no matter how well you pinched it. I learned that the hard way more than once.

Same seam problem here for ages, and what actually fixed it for me was a plastic dough scraper. Sounds random but I use it to flick the last flap of dough over and press the seam down in one motion, so my warm fingers never squish the dough and pull the seam apart again. Fingertips kept undoing the work because the dough was so slack it stuck to my skin.

Also worth checking your transfer to the stone. I was lifting mine off the cloth by hand and every one that got manhandled opened at the seam. Rolling them off a thin board seam side down now, barely any touching.

Quick update - I finally figured out the issue. I was focusing too much on the oven setting, but it was definitely the shaping. Using a plastic dough scraper to flick and press the final flap down in one go fixed it. My fingers were pulling the seam apart when I tried to pinch it. The loaves are coming out perfect now.