Tartine bread problem

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Can anybody tell me why this sad layer under the crust developed? Bread (Tartine Country) is definitely fully baked and the crumb is just perfect.

 

I can't tell, is that ring under the crust gummy? I can see a darker ring just inside the crust. 

Bread texture looks OK 

Baking at what temps for what times? 

I've only been baking the country loaf for a couple of months, but I have had problems with gumminess. I started to do a few things differently and have seen improvement: First, I do much gentler turn and folds during bulk. Second, I've watched a few videos of professional bakers shaping their loaves and that helped tremendously, because I couldn't figure it out from the photos in Tartine. Third, I do longer bakes now. I do 20 covered in a large BK Dutch Oven, and 25 uncovered (both at 450). I've lost the gumminess and have larger, more consistent holes throughout. It's not even close to perfect, but it is delicious and lighter and springier than it used to be.