Thanks for the interesting conversation. I've resolved the issue however, in a roundabout way and by cleaning up my code.
The UDP communication code in question is a model written in LabVIEW running on VeriStand, which is essentially a bunch of LabVIEW RT timed loops running on a Phar Lap target. So the model essentially opens a UDP connection, sends the message, then closes the connection. This is done at a rate of 50Hz. I changed the code so that it opens the connection at the start of the test and the model just continuously sends the message at the 50Hz rate, then at the end of the test I close the UDP connection. This resolved the issue because now the ARP is not sent anymore, like at all. Well, maybe at the start? But I haven't checked that, so I should do that later.
The explanation I can come up with is, I was closing the connection every 20ms, so the OS considered the port closed, so it took that time to do the normal housekeeping of sending the ARP? But since the port was always open to a known IP address (and thus mac ID), it didn't need to do the ARP. I don't know, just my guess.
Ok, so bring on the comments about, why I would keep opening and closing the port. Bring it on, I can handle it... 😀