You said " famicom disk system", did you try the rom game. If the rom has the same issue it may be a mapper most emulators do not emulate yet. If it is just the disk system, my guess is its specific to disc emulation.
Either way if two rips of the game act the same on two emulators, its not fixable.
yep but Fhorse is developer im not joke.(nesdev and github )
he is developer and owner of punes the same dude which post about new mappers.
so i recomend punes its still going foward and add new mappers.
thats why i write punes im dont know nestopia and fceux have new mappers and fix.
because Nestopia UE is fork of orginal Nestopia which ended at 1.41(unofficial)
Fceux suck whith FDS can emulate only few basic mappers *more popular* .
so anyway i will worship Fhorse work and him because of his work but i dont like GUI ugly for me test shows attention :
Punes is faster than Fceux and Nestopia !
its not joke nestopia sometimes pretend to lag.
and load games slowly the same fceux im dont know how much nanosec but still it faster.
im not use punes often because just like i say i dont like GUI .
so i still use Nestopia with mappers which suck but at least sound its good ("soft"),
from fceux i hear just harsh noise so i dont like it even on higest settings (its awfull so squared so high pitch even on default).
dont belive me ? try the same rom on nestopia and fcux then set to lowest settings doesent matter what you do in nestopia sound will be still clear and soft on fceux not ).
thats all there is more emulators but they are crappy by design and not fixed and often abadoned by developer like nesticle.