It’s not a bug, it’s a new feature:
Managing the loopback interface
Because of the loopback interface’s somewhat unique characteristics, NetworkManager traditionally treated it specially. That basically meant it ensured it’s up and that’s about it.
In the new version of NetworkManager, loopback interfaces are less special. While it’s still a little special – it will be configured even if there’s no connection profile for it – it’s now possible to actually activate a connection profile on a loopback interface. It behaves a lot like on an actual physical interface with some interesting properties. With the loopback interface always guaranteed to exist in one instance, a connection on a loopback interface can be used to specify configuration that’s always supposed to be applied.