Sourdough

Hi!
I didn't have this problem baking home in a dutch oven. Now when I've switched to the combi oven plus baking stone, sometimes the crust separates from the crumb. I use banneton basket to proof, so my breads land "up-side down" in the oven, so It means that the big wholes form in the bottom of the proofing dough.
The dough is 78% hydration, no knead,  1X stretch and fold, Stone preheated to 280C, baked in 240C for 25-30 minutes, internal temp >95C, weak flour (12% protein)