That helped.
I think I know what the issue is / was. It seems like Traefik isn't passing the host header...? When I configured the target service to accept srv-tfs-p03.domain.local as a valid domain name then it started to work.
Which is weird, because, as stated, if I set up a HOSTS file I can use the devops-test.domain.com just fine, but Traefik couldn't use it.