A broken poly lock. You checked coil binding and it was okay. Did you check retainer-to-guide clearance inside the spring? Rule of thumb, I believe is 0.060 inch of clearance between the two items.
Could also be an over hardened, read overheated at time of heat-treated, p/l at its manufacture, and not come up again. Could the tightening of the setscrew caused too much pressure exerted onto the small area remaining of the p/l's body? Maybe, but, I would think the threads would have stripped out from either the setscrew or p/l before splitting the p/l.
I'd probably buy a new set of p/l's and install, throw the rest away. One might use a microscope and inspect the same area of the side of the p/l for cracks just to see if the others were going to break.
Too much lift on the cam? Even though you have no coil bind, the lift action of the lobe multiplied by the rocker arm ratio may have swung the r/a past its designed arc at the fulcrum bearing causing the pushrod to continue to push up on the arm past its arc end.
Check to make sure the pushrod(s) is/are straight by rolling on a sheet of glass or mirror. If its bent, something has stopped the opening motion of the r/a.
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