That is a symptom of the wild yeast taking too much time to raise the bread. Until the starter is active enough to raise the bread, use a teaspoon of commercial yeast in the recipe. If you do that, add the yeas in 1\4c of warm water(proof the yeast first) and add it to the sponge in the morning when you make the bread.I have the bubbles in my dough. Just like your picture. But the outside also has the bubbles. I did them in loaf pans because my husband wanted them in loaf form.
You will need to do this for 6 months to a year.