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'How did you determine the scope is not holding point of aim/impact?" I really don't understand that question. My simple answer is because it doesn't.
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I think the question is "How do you know it's the scope and not the wind, your trigger pull, a flinch, a stroke, or a heart attack causing your problem". To some shooters the last thing they might blame would be the scope, and that's not a bad thing because good scopes don't often go bad. But that doesn't mean they can't.
In fact I have a Sightron SIII going back for repair today for the same problems you're experiencing. To the doubters, I'll say this.
Folks who shoot in competition requiring high precision, such as F-Class, Palma, or Bench Rest and who also have some experience usually know if they've launched a good round or not. Since nearly every shot MUST be fired properly using a consistent procedure to arrive in the winners circle, "pulling" one or shooting a flyer is not only a rare thing, it's also pretty easy to tell you've goobered up your shot the moment you launch it. Folks who get excited if they hit a paper plate at 50 yards half the time may not have the feel for this, but precision shooters usually know if they've made a mistake well before the bullet arrives at the target. Nobody is going to return a scope based on one errant shot anyway.
Then there is the wind. In my case I have the luxury of doing nearly all my testing in near zero wind conditions. A wind too light to feel or too light to spin my meter is not going to suddenly cause a 2 MOA windage error at 100 yards. In short, if there ain't no wind................. it ain't the wind.
Most competitive shooters know how their "good stuff" shoots. If you average, for example, .350 MOA at 100 yards you would not be surprised to go to the local range and fire 10 or 20 five-shot groups all between .500 MOA and .200 MOA. You might shoot one group in the 6's or 7's but that would be unusual. What you would NOT expect to see is a bunch of good 5 shot groups followed by a group with 4 touching and the 5th shot impacting 2 or 3 MOA off to one side. At first you might think that you've loaded a bad round. But if it happens 10 or 20 or 30 times, you have to start thinking that it might be an equipment problem.
In my case, and I suspect in the case of the OP, the next step is to review your reloading procedure, make sure the scope mounting is secure, action screws OK, re-check the scope parallax, and so on. But after I endured a couple of range sessions with the same problem I mounted a different scope. The problems instantly disappeared and my groups returned to normal.
Bottom line: Nobody (well hardly anybody) would ship a scope back after firing one errant round. But after exhaustive testing and by the process of elimination, it's possible to be pretty sure something inside your scope has failed. Considering all the tiny moving parts in a modern multi power scope it's amazing they survive as well as they do.
