Without knowing your process, it's hard to say for sure. When I made loaves like that it was because they were under proofed.
Can you share you recipe and process, especially around fermentation times?
I used food geeks beginner sourdough recipe. It’s a 70% hydration bread. 81% bread flour and 19% whole wheat flour. I autolysed the flour for about an hour, then mixed in the Levain and salt and let it sit for another 30 mins. I performed 6 sets of stretch and folds because I couldn’t pass the windowpane test until then. I then let it sit to bulk ferment for 2.5 hours. Preshaped, then sit for 15 mins, then final shape and in to baskets in plastic in fridge to retard overnight. Baked in a preheated oven with pizza stone and Dutch oven at 500 for 15 mins and then uncovered at 450 for 23 minutes.
I'm sure I sound like a broken record on this forum, but nearly every new sourdough baker has loaves that look like this.
Most likely the culprit is your starter. My bet is it's too new and/or not active enough. When your starter is at least doubling within 8 hours (ideally less), you will be more successful here.
I should add that the stretch and folds were done every 30 minutes.
How did the dough look at the end of the bulk ferment? Was it puffy, jiggly and with some nice bubbles? It's important to make sure you get to that point before you retard/final proof in the fridge. So, better to judge that by appearance rather than a set time in a recipe. Also, it helps to make sure your dough temp is warm enough during the bulk ferment, so using warm water and leaving the dough in the oven with the light on (oven off of course) helps maintain a nice warm dough. The loaf also looks a tad under baked, so making sure your oven and DO are scorching hot should help with oven spring on a cold dough--I usually bake in a DO at 500 for 25 minutes covered, then reduce heat to 450, take lid off, and bake another 20 minutes. But, the 'mouse holes' suggest under proofing. You might reduce the whole wheat a touch and see if that helps.
Totally agree. My mantra lately has been "let the dough lead!". I had been trying to force the bread into my timeline instead of allowing it the time it needed at various stages.
I have had good luck with a putting the dough in a clear plastic tub and marking the height. I then let it rise 75-100% and then do the shaping. Then, I leave it in the banneton until it is just about to pass the poke test and then into the fridge. Results have been so much better!
100% definitely underferemented in bulk. My guess is, like almost every new sourdough baker, the starter just isn’t active enough.