Hairlines at the quarters do not jam upward because the farrier didn't place an arch in the wall the bottom. That is not the cause.
They jam because the heels are run too forward undef the foot, causing the upward forces from the ground to be directly under the jammed area and heel bulbs lacking support far enough rearward.
Unsupported hairline at heel goes down while the loaded hairline above the 'run forward' heel buttress goes up. simple.
If they jammed because the lower edge of the foot was left flat , then all horses with shoes would have jammed quarters and they do not.
The ones who have the heels trimmed correctly do not jam the hairline, the ones with overgrown or under run heels do. It is very predicable.
I have "un jammed" quarters all the time in shod horses , with the lower edge of the foot flat, simply by trimming the heels back where they belong and loading the back of the frog. Whether shod or barefoot .
Just look at any foot you see with jammed quarters, where the heels have nt been corrected yet there heel buttress will be directly under the highest part of the hairline jam, every time. it is extremely predictable. The reason barefoot trimmers fix the jams is it because they arched the quarter on the bottom. It is because the trimmed the heels. But they don't even realize how they they fixed it .