I think if the distributor was mashed down hard enough against the block to shear a roll pin that there would be other symptoms and here's why I think that....
I had an engine builder send me a distributor to use on a Windsor once. Trusted him to put a gear on, I didn't check it. Primed the pump, put the distributor in, fired it up on the dyno, and the distributor starting spinning as it was starting to wad up all the plug wires. I grabbed the distributor with both hands to hold it while I was screaming at my dyno guy to "SHUT IT DOWN!".
The engine builder had put the gear on and didn't get it on at the right dimension. It had taken all the end play out of the distributor and it was enough that it was starting to spin the whole shooting match.
We are talking different situations. A gross difference in height, enough to shear a pin immediately, or bind the distributor, would likely do exactly as you saw, not sure how it couldn't, and it would blow my mind if I saw that on the dyno LOL
Consider just less than zero clearance between the gear and block, in fact, it could be that you even have a little cushion with the distributor seal, which of course isn't supposed to locate the distributor. As that engine runs, it's slightly lifting off and on the block locator, or trying to, putting pressure on the pin and more so as the gear heats up. It's like working a piece of wire until it breaks.
Gross binding versus incorrect clearance....I can't say that WAS the issue with this one, but the fix is all the same anyway. Get the gear right, or check it is, and see if any carnage in the pan
In the end, not the same parts, but we ran into this with rebuilt distributors as long as we had the old shop in MA. Cardone was the worst, random gear height installation, especially FE and Windsors. Much smaller diameter pin in a stocker, but the initial failures, before we started checking every one, were usually days or weeks down the road. When we finally figured it out, every one we checked was deep. We told Cardone, not sure they cared LOL but it does happen