Our business (Tock) is also experiencing this exact situation with the discovery library that Epson provides. It works fine when installed for iOS 13, but when built with the latest XCode for iOS 14 and installed on a physical device, the discovery portion no longer works.
What is interesting is that everything works correctly when installing Epson's own ePOS_Discovery sample app that comes with their latest iOS SDK download. We build that with the same Xcode and install onto the same device. Once the Local Network Privacy option is agreed to, it can find the Epson printers without issue.
But, for some reason, when this library is used within our application, after agreeing to the Local Network Privacy option, we are unable to discover printers and were able to do it before without issue. Our app in the AppStore which wasn't built with the latest XCode can install onto iOS 14 devices and works without issue as well.
So, yeah, a big mystery here. I'll let everyone know if I find anything interesting, but if anyone has pointers to potential fixes, please let this thread know.