Hello and welcome to the forum. Your question, and similar ones, have come up quite a bit. Allow me to summarize and link to some info:
The new Roon ARC feature in Roon 2.0 opened a port on your router and is forwarding it to your Nucleus. This is for access to your music from your phone when outside your home, using the Roon ARC app. As the port is open, it gets automated scans like every one of the billions of open ports on the internet. However, only the Roon ARC app can actually connect to the port.
About ARC:
About how Roon enables the access. In your case, as this happened automatically, your router has the UPnP service enabled. This informs devices like the Nucleus that they are allowed to open the ports they need:
Security discussion:
If you don’t want this, disable UPnP on your router.