Forensic help needed ..

Hi everyone, glad to be back. Need your help for the below frustrating mishap when trying a new recipe 

Getting back to sourdough backing after a while, and baking a 71% hydration pretty standard sourdough BUT got no oven spring and a kind of "dent" ( see the red arrow) in the crust. The worst is the crumb with a huge hole in the middle and near the crumb.  What went wrong?  

Process :

Mix flour and water in stand kitchen and autolyze for 45 minutes

Add starter and knead in stand mixer for 3 minutes, low speed. Add salt and knead for extra 6 minutes  (result - very sticky dough, limited window pane)

Bulk fermentation -  5 hours at 23 C ( see pic) , 6 quite extensive folds. ( result - nice stretchy dough , not bubbles and limited growth (about 40% in volume) 

Pre shape - standard. 

Shaping - Batard , stitching method. 

Retarding -  1 hour in room temp, 11 hours fridge (result - not much growth in volume , quite low profile, feels good and not slacky, easy scoring) 

Baking - 260 c oven with stone + boiled water for steam - 20 min covered, 25 min 240 c uncovered