I think the problem is the guide plates causing the roller tip to be not centered on the valve tip. This in turn is imparting a twisting action on the rocker, & that has flexed the studs & they have snapped.
Many times I have had the above problem of misaligned guide plates. I cut the plate in half, reposition the halves for correct roller tip alignment & weld the halves in the new position.
Other possibilities: springs are coil binding; retainers hitting guides at full lift; material fault with the studs, have seen them snap like that.
Looking at the pics, there seems to be enough movement for the stud, zero to max lift, within the rocker slot; so I do not believe the stud is contacting the slot. In the pic, the mark looks like a material flaw.
