This is the first time I hear about removing the breech plug to inlet the barrel. Is this a common method? Is it recommended in said kit's manual? If yes, perhaps the breech plug is not torqued fully initially in expectation the kit builder will torque it once the inletting is done.
There are few reasons why I never remove breech plugs in any of my muzzleloaders. This is just my opinion.
First is that all my guns were proofed (in Italy, in UK, in Belgium) and the way proof marks are placed leads me to believe barrels would be considered out of proof if breech plugs were removed.
Second, on some guns like my classic side by side smoothbore, kodiak double rifle, howdah etc. Breech plugs are not just screwed in. They are hard brazed in.
Third, breech plugs are supposed to be torqued properly and they need to be at a specific rotational position resulting in proper seal on the barrel shoulder. Repeated removal and reinstallation will lead to some thread degradation that will require either turning the plug past previous point messing with its position, or it'll be slightly looser. In a round ball rifle or a smoothbore that may not be a big deal, but in a fast twist, small caliber rifle that shoots long heavy bullets that may cause issues. It is not beyond the realm of possibility high pressure rifle like that could push powder fouling into slightly loose threads corroding them in time. Loosening even more. Eventually perhaps leading to failure.
Finally, there are many ways to remove a bore obstruction without having to remove the breech plug.