Ok, the issue turned out to be that the iOS device lost internet connection and therefore couldn't verify the signature to run the app.

I found out that as of Xcode 12.2 SwiftPM do not work reliably with playground. Currently, I can't make it work. I use 2 slightly different ways to marry a package with playgrounds: by connecting them through a workspace and through a project. It worked once, and to make it work I had to restart Xcode. But since I decided to replace the package, I was unable to make it work ever since. At this point I'm not sure if I'm doing something wrong or is it an Xcode bug. I've decided to stop trying and give up on playgrounds until SwiftPM support will become dead simple. Too many hours wasted. Build active scheme is turned on.