Mirror Networking is an Asset that helps you making your game multiplayer-ready. You can do very high level things and then on start or building, it has some weaving mechanism inside that adds some additional networking code to the classes which do the rest of the communication.
You can find Mirror here: Mirror | Network | Unity Asset Store
I don’t believe that it’s mirror’s fault but somehow Unity. There are some other older threads in the forum about similar problems like this one: Script changes don't compile