varnishadm or varnish in weird state

[John Cherouvim]

I have 2 Ubuntu 16 LTS servers provisioned using the same ansible
playbook and they both run varnish-4.1.1 with the exact same
configuration. The first server is production (receives a lot of
traffic) and the other one is staging (minimal traffic). They both have
~20 days of uptime with varnishstat reporting an 11 days uptime each.

On production I get weird results for trying to list the backends. Some
times I get this:
$ sudo varnishadm backend.list
PONG 1503057040 1.0

And some other times I get this:
$ sudo varnishadm backend.list
Backend name                   Admin      Probe
8440ffbd-e1de-4827-9f83-9096f5a97bf1.www probe      Healthy 5/5


but on staging I consistently get the following, which is what I've been
used to seeing in all other environments I've used varnish:
$ sudo varnishadm backend.list
Backend name                   Admin      Probe
boot.www                       probe      Healthy 5/5


My /etc/varnish/default.vcl starts like this on both servers:
> vcl 4.0;
> backend www {
>         .host = "localhost";
>         .port = "8888";
>         .connect_timeout = 60s;
>         .first_byte_timeout = 120s;
>         .between_bytes_timeout = 120s;
>         .max_connections = 256;
>         .probe = {
>                 .url = "/health-check";
>                 .timeout = 15s;
>                 .interval = 5s;
>                 .window = 5;
>                 .threshold = 2;
>         }
> }



For that kind of configuration I've never seen that hash like backend
name, nor that behavior where it randomly shows that or the "PONG" response.

Doing a "service varnish restart" fixed this.
But does anyone know why did this happen in the first place?

thanks