The valve heads are brittle. Clamp a normal steel valve in a vise by the stem, grab the valve head with a wrench that has some leverage (large channel locks, pipe wrench, etc) and you can bend the stem. Do the same with a Cleveland valve and the valve head snaps off.
They develop cracks below the section where the valve head is welded to the stem. The cracks start-off very small, microscopic, and grow larger over time. The cracks lead to the valve head breaking completely off the stem and bouncing around inside the cylinder while the engine is running. Often putting holes in pistons, cylinder heads, and/or cylinder walls.
The question is what starts them cracking? For that I have no answer. Now the Cleveland is not the only gasoline engine that has had this problem, others have been plagued by this same malady. I've read all sorts of theories about what starts them cracking ... constituents in fuel for example ... but there is nothing definitive that I'm aware of.
I remember walking into a wrecking yard circa 1974, and inside the office they had not one but two Cleveland engines sitting side by side in the corner. Both engines had one head removed, exposing a cylinder with damage from a broken valve head. Both engines had come in within the last month. They were almost new, no more than 2 or 3 years old. On the other hand I know several guys who put 200,000 miles on a Cleveland without a problem. There are Panteras still today, like yours, equipped with the OEM valves. I know one guy who had a valve head fall off the stem while his Pantera was warming up, idling, in the driveway. I also know guys who raced with the stock valves. RPM doesn't seem to have anything to do with valve failure.
Nobody can guarantee you that out of the 16 valves in your car's engine, one of them won't fail. But nobody can predict if and when they will fail either. A failure is usually quite expensive to repair as at least one of the castings will end-up with a hole or crack in it. Each day you start an engine equipped with the OEM valves is a toss of the dice. That's really the best answer I can give you.
The only sure way to avoid failure is to replace the valves. You don't have to install an expensive set of stainless valves, a good quality set of steel replacements designed for 4 bead locks will suffice for applications up to 6200 rpm rev limits, at a much lower cost than the stainless valves. The last time I checked Speed Pro (Sealed Power) had the 2.04 intake valve and the 1.65 & 1.71 exhaust valves in low cost steel replacements, but not the 2.19 intake valve. But I haven't done an exhaustive search.