Followup:
I spent 30 minutes looking into this and freaking out because my own copy of this app wasn’t working with UDP either; I verified our BPF maps on the worker server my Fly Machine was running on, did a 5 minute detour into how fly-proxy handles the flyio-debug:doit header when no TCP servers are lit up for an app (spoiler: it doesn’t work), found an AT&T Fiber Chicago routing issue (my packets are all going to Newark because AT&T and Comcast have colluded to make my life miserable), which Peter is looking into now (sorry Peter), isolated the weird EWR edge my UDP packets are hitting, did packet-level diagnostics there, couldn’t see my UDP packets, freaked out some more, then realized I was missing the -u in my client nc connection.
Once I added it, everything worked fine.
Again: not ruling out anything broken in your app. All I ran on the serverside was nc -l -u -p 5000 -s 172.19.5.195. Note that I looked up fly-global-services and used the IP directly, because I don’t trust nc, which was written in the phlogiston era of network programming.