Batard dough flattens out in the oven, need help
I recently decided to challenge myself by giving up my Römertopf (or Oblong clay pot) and decided to use some kind of rectangular Dutch oven like this one in order to bake my batard-shaped sourdough breads :
(I use it upside down such that the lid gives me a large flat surface)
Where I can easily fit 2 kg of dough in the shape of a batard.
The issue being that now, I have nothing that retains my doughs from flattening out after putting it straight from the banneton on the lid of the dutch oven. I then put the lid and cook for the first 50 minutes with the lid on but when I remove it, I always end up finding some kind of flat pancake (Photo below), that remains delicious but which he's nothing from the likes of the previous batards I would bake in my Romertopf claypot. My hydration level is usually between 68 and 72% and I proceed the following :
Mixing : 1kg of flour (200g of whole wheat, 300g of white flour and 500g of semi whole-wheat flour, what we call here bise flour), 200g of double sized sourdough started (fed with 100g of rye flour and 100g of water), 20g of salt and hence 680g approx. of water.
Then every 30 minutes after mixing, I do a set of Stretch and Folds. After 2 hours of Bulk fermentation and 3 S&F, I leave my dough for about an hour at room temp (round 22°C), then preshape, shape tightly (in the likes of King Arthur) and then put it in a large oblong 26cm banneton.
After 8 to 24 hours of proofing in the fridge (7°C), I put it in my preheated dutch oven and that's where's the rub, the dough flattens during the first minutes of baking, ending up like a flying saucer.
Any idea what I am doing wrong, is it the tension that is not good enough or maybe the fermentation process that isn't long enough ? My cooking vessel ? Or maybe even the flour content that is too "whole", though I doubt it's the issue since plenty bakers in the surroundings (Switzerland) are able to come up with brilliant loaves with 100% bise/semi whole wheat flour.
Anyway, some help would be great.
Greetings from Switzerland and thanks in advance.