For completeness, adding here the info we talked about in the issue (https://github.com/containous/traefik/issues/5419):
- There was a bug, but it's being fixed.
- Even without this bug, one could end up observing the reported behaviour if one does not specify the network for their docker services, because swarm might balance containers between different networks, which would make the servers URLs "fluctuate" in the config. TL;DR: please do specify the network for each of your docker service.