We've run into this twice now. Existing proposed faulty patch here: https://gerrit.wikimedia.org/r/#/c/276529
The problem is that the backend name (not hostname, but labelling key) in VCL is built from only the first label of the actual backend hostname (or all of the digits of the IP, if it's an IPv4 address). This should include at least the next label (for datacenter name), if not the whole fqdn. It should also include the port number. The complication is that we have to do this transformation in two places identically: once in puppet/ruby, and once in a go template, and the go template lacks regex functionality.