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Hi Rowdy and welcome to the forums.
To me, this doesn’t seem like an inaccurate fault since as you say the Ethernet cable is unplugged. The controller is probably doing a “heart-beat” type check (or more likely sending an ARP packet out periodically) to see whether its network connection is active or not.
Then as you say, the fault goes away when it detects that it’s connected to a network. It doesn’t seem to me that this is something you should be concerned about, in fact, you may want to consider monitoring this fault in your logic to annunciate it in the event it does occur when you least expect it!