20 plus years of muzzleloader recoil isn't exactly terrible performance. Considering that there may be something about muzzleloader recoil that stresses the scope slightly differently than what it was made to deal with.
I don't know that there is, seems like it shouldn't matter, but sometimes, it does.
One thing I do know is that some airguns will kill scopes that can live on elephant guns, in fairly short order. And one sees scopes mounted on the slides of semi autos fail at a different rate than when mounted on things that don't move like that.
ITs all dependent on how the scope is made and what stresses it is built to take, and what direction the force is applied.
The best stuff will survive thrust from both directions equally. The best stuff is expensive. Some cheaper stuff will serve well enough, long enough, some won't.
Some time back I bought a cheap ($35) red dot sight and put it on a .45-70 Contender, mostly just to see how long it would last. SO far, its made it a couple hundred rounds and wacking a friend of mine in the nose with no apparent damage or change of settings.