Is Java UDP communication broken on macOS?
No - UDP isn't fundamentally broken on macOS (or any of the other AppleOS)

The definition of UDP is unicast data packets and due to the send and forget nature of the casting, delivery of these is sometimes jokingly called unreliable data packets. 

As a troubleshooting step, my first triage step would be to establish that the switch and router are actually delivering the packets to the macOS interface. Wireshark or a management tool on your network would be excellent tools to record these packets to see if they are being delivered.

I wouldn't worry about people saying TCP is more or less reliable than UDP since UDP has many technical advantages for streaming performance and you are on the correct path focusing on a specific java application that consumes data on a specific Mac in your network. Lots of your details are good, but it's kind of like detailing the pressure in the tires and the color of the paint when you actually need to know if the driver took a right down your street.

Are you able to run Wireshark and verify the packets or get access to the network logs to verify the sender and also you might need to do a little research on UDP and ask a follow on question if this actually is more about networking.


https://stackoverflow.com/questions/5970383/difference-between-tcp-and-udp