Ran into this online about sourdough blistering. Now I get to poke
around and see if it makes sense.

"This is because acidic conditions created by sourdough breaks down
the gluten near the surface. As the bread bakes, steam is created and
it escapes through mini-chimneys all throughout the crust where the
gluten has been weakened. Those mini-chimneys end up looking like
blisters."

https://wheatbeat.com/blisters/