A .459" bullet is undersize for most Trapdoors.
Moyers says their .45-70s are BHN 13 but I suspect it is still antimonial lead.
You had been shooting jacketed bullets and probably still had some copper deposits.
All that and a rapid loss of accuracy adds up to a badly leaded bore. It is going to take a lot of cleaning and then proper loading.
There is a book on loading for the trapdoor by Pat and Spence Wolf. It would pay to read it.