I could very well be wrong, but I don't believe that Nestopia UE is in the repos, see here: https://dev.getsol.us/T1050
So as such Solus doesn't officially support it. I do see they have a flatpak version of it, which is what I'm assuming you're using. However, flatpaks are software that is something you'd have to take up with the flatpak maintainer. Not that it may help, but it never hurts to share, if you check out fceux or retroarch, those are in the repos and should work just fine. Hope this helps a bit and good luck!