Me either, but I use to run into guys that had that problem a lot with center fire rifles. I've never had that problem with them either.
Sometimes I wonder if it's not the shooter having first shot jitters.
In all honesty I have seen this with centerfires too but not mine!
I am convinced that the variables that contribute to this are nearly as infinate as the universe!
However, in saying that the most common connection is that the first shot is from a squeaky clean barrel! I emphasise squeaky !!
A friend of mine use to clean his 270 with a concoction of chemicals that I am sure it aided all my hair falling out!
His first shot was always off by a huge 3/4"! I on the other hand never fussed that hard, just prevented rust. I never had a first shot out of group or off zero.
I also don't get panicky about my muzzleloaders. Yes, the salts are all dissolved with water but should I insert a white patch it will come out brown or black but not wet from water, wet from my patch lube that was applied to the warm barrel. In other words my barrel is already fowled to some extent just no corrosive elements are present.
I don't believe the frontiers men cleaned their guns as many do today. They were shot every day nearly and a thorough clean every day would just not be practice. They knew that animal fats would protect perfectly fine the bore and outside of their guns. They had so many chores that salts were the last thing on their minds! Why one of the reasons a gun was kept over the fireplace was to keep it dry, why? Because without water the corrosive salts are harmless, my own findings are I can bring a damp fouled gun in the house and place it by the fireside so it dries out and for days the gun suffers no harm!
There is so much emphasis these days on corrosion, rust, damage etc that it's almost become an invisible torture stake that many drag around, I did too until recently!
B.