You owe it to your horses to inspect your bits
carefully and often!
The very sharp edge (orange arrow) on the relatively soft copper roller next to the center joint in this simple snaffle was created by the roller rubbing on the swell of the joint (green arrow) over time....years probably. It was really quite sharp. Sharp enough to have a horse react negatively. That surface should be round and smooth. The other side of the bit was in the same condition, forming a sharp pinching edge between the two. Fortunately, this is easily fixed with a small file.
Here's the same bit bit with a slightly modified construction in the problematic area. Note the shoulder of the joint adjacent to the copper roller (red arrow). It's not tapered. This will keep the copper roller in it's original shape.