The scarring first started over winter, the cacti sat in a west facing window. I did not water them during the winter, and that's when the original scarring happened. It is discussed further back in this thread somewhere.

I didn't think anything of it as it stopped come growing season when it got more light and water. This time though it seems to be spreading even during the growing season. It is on all sides of the cactus, so the shading I dont think is playing a part. They usually are in full sun for most of the day anyways unless there is rain in the forecast. The past couple storms we got I didn't move them right away and they got a lot of water, but it seemed to just make them thicker and grow faster, no damage from what I could tell.

So I am stumped as to what the problem is. The sections that are scarred are also kind of thin for some reason, but the rest of the cactus is nice and thick. Could it possibly be from poking itself with its own spines? They definitely do poke themselves quite a bit, and the sections that dont get poked are the parts not showing scarring.

The cutting I have in the other pot doesn't seem to be experiencing the same thing, and it's from the same plant. It looks nice and healthy. It's also a single stalk with a tiny basal pup, so no being poked by its own spines yet.

Edit: I also noticed the arms are growing in a way that they are bending and twisting away from the main stalk, as if they are trying to avoid being poked by spines. Maybe it's more light induced, but like I said they are in full sun most of the day so they wouldn't need to reach in any specific direction for sun like that.