This looks like an excessive amount of flour worked into the dough when shaping - the spots do absorb humidity from the surrounding dough, but remain dense and hard.
To prevent this, be mindful of how much flour you add to your bench and how you shape your loaf. Especially with rather wet doughs, the impulse to flour generously to prevent it from sticking is obviously there. And while you can and probably want to flour what’s going to be the outside of the loaf, you should be careful about not working the floury parts to the inside. There are a few different methods how to solve that, if you search for shaping especially wet dough, you will find various methods that can also be used for medium hydration doughs. For example, Foodgeek has a nice video where you can see the shaping of a boule completely without bench flour and a batard with minimal flouring.