It might have just been cold overnight? You could try keeping it out of the banneton overnight, then put it in the banneton first thing in the morning and give it 4 hours or so. I think this will give gluten to develop before you try and get tension in the dough
It’s a fairly simple recipe;
Starter mixed with water, add flour, lyse.
Add salt/water mix 6 x folds every 30.
Rest 1 hour
Shape
Rest 30
Proofing basket.
Proof over night back in the morning approximately 12 hours.
The less time it proofs for it stays in its shape out of the basket. Any longer and it goes on the hot try and splays out flat as a pancake