One Big Mouse Hole

I am trying to bake a Tartine country loaf.  Unfortunately I’m getting loaves that are one big mouse hole.

Vigorous starter fed twice a day and passes  float test. 

For this recent loaf A/C was set at 26 c during bulk rise and then loaf retarded over night in fridge. Baked straight from fridge.

I have tried bulk rise without A/C and leaving bread out at room temperature for 2 1/2 hours from fridge before baking but with identical results. One great Mouse hole.

Be grateful if anyone has any ideas what I am doing wrong.

Thanks