docker swarm mode uses apparently wrong interface for UDP packets

I might be naïve, but I'm seeing this and it really looks like a bug, not a feature.

I'm running several services in a swarm, and all the exposed ports show essentially the same behavior. The service address is 10.0.0.81, and the node address is 10.0.0.18.

When I send TCP service requests to, say, port 443, they are received as packets like

(...)
13:46:29.762665 IP 10.0.0.81.37650 > 10.0.0.18.443: Flags [S] (...)
(...)

But when I send UDP packets to, say, port 10000, they are received like:

(...)
13:50:12.910212 IP 10.255.0.2.60602 > 10.255.5.25.10000: UDP, length 4 # those are npings
(...)

I would expect the same interface eth0 to be used for TCP and UDP services.

I'm using

$ docker version
Client:
 Version:      18.05.0-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   f150324
 Built:        Wed May  9 22:16:13 2018
 OS/Arch:      linux/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.05.0-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.5
  Git commit:   f150324
  Built:        Wed May  9 22:14:23 2018
  OS/Arch:      linux/amd64
  Experimental: false

Start by reproducing the reported TCP and UDP packet paths in a Docker 18.05.0-ce swarm using the addresses and ports shown. Compare how swarm mode routes each protocol and verify whether UDP should use the node interface like TCP; done means identifying the cause and correcting or clearly documenting the behavior.