The OP guessed right. As the vent in a flinter enlarges the accuracy suffers, but in a different way than with a caplock.
As more of the flintlock vent opens, more gas pressure is lost through it. So over time, MV is reduced. Now out to 50 yards, the shooter might not see this for very many years, but out at 100 yards...the lower MV will cause the point of impact to drop. So the shooter may get a good group but "the sights are off". So the flintlocker might adjust his front sight post, making it shorter, and not knowing that what really is needed is to replace the vent liner with one with a smaller hole.
The caplock does not have the powder as close to the outside air, as does the flinter. Remember there is an air gap between the cap, through the nipple and the drum, or through the "snail" breech, and the main powder charge. As guessed, the hammer holds down the cap, and the breech explosion compresses the tiny bit of air in that gap, but it's consistent. When the blast starts to lift the hammer, it then gets inconsistent. Consistency is one of the very first rules of accuracy. AND like the flinter, the caplocker might not notice the problem at 50 yards, especially if the caps are damaged but the hammer only moves off the nipple for a fraction of an inch and returns to rest upon the nipple. At longer ranges, however, the inconsistent MV will amplify the group changes on the target, and it will be readily seen. Might take years for a hunter to see it, but a target shooter will likely spot such a change much sooner.
What's old is new.... modern ammo hand-loaders will get a pressure loss and voila, a flier hits the target outside of the desired grouping when a primer is pierced. I won't go further as this is not a modern rifle forum nor website.
LD