If this is true, then it IS a problem.
I had the same problem with a semi-custom made rifle that was converted from Cap to flint about a year and a half after I bought the gun. When we pulled the Breech plug, we found lots of crud, and worse, LOTS OF CORROSION in the gap in the front of the plug and behind the shoulder. I had shot the gun a lot, and had cleaned it religiously, but could never get a totally clean patch to come out,no matter what I did.
I consulted more than a dozen experienced shooters at my club about the problem and NO one knew THEN what the problem was. When the man who converted my gun from percussion to flint removed the plug and found out what the builder had done, he was furious. He bought a new plug, hoping to close the gap. It did, but not enough, and he could not find a breech plug that had a longer plug on it. So, he ended up making a metal spacer on a lathe, that fit between the face of the plug and the shoulder, to see the back of the barrel properly.