Percentage Based Routing and NOSRV

I have setup a percentage based routing configuration by splitting two different clusters into two different backends through some redirection back through front ends as seen below. (note: this is my first time configuring routing to facilitate a slow cutover to a new cluster, if there is a ‘smarter’ way I am open to hearing about that too.)

    log      local2
    pidfile           /var/run/
    maxconn           30000
    user              haproxy
    group             haproxy

    mode              tcp
    retries           3
    timeout queue     1m
    timeout connect   1m
    timeout client    1m
    timeout server    1m
    timeout check     10s
    grace             10000
    maxconn           30000
    balance           roundrobin
    log               global

frontend frontend_a
    bind              *:5000
    default_backend   backend_a

backend backend_a
    balance           roundrobin
    option            tcp-check
    server            cluster1 localhost:5001 check inter 10s weight 50
    server            cluster2 localhost:5002 check inter 10s weight 50

    bind localhost:5001
    default_backend backend_b

    bind localhost:5002
    default_backend backend_c

backend backend_b
    balance           roundrobin
    option            tcp-check
    server            backend_b_server1 check inter 10s
    server            backend_b_server2 check inter 10s

backend backend_c
    balance           roundrobin
    option            tcp-check
    server            backend_c_server1 check inter 10s
    server            backend_c_server2 check inter 10s

I am seeing the logs properly identify frontend backend/server name upon entry into the system

frontend_a backend_a/cluster1

but when going through the next layer it loses the server name, for some reason I get

frontend_b backend_b/<NOSRV>

The balancing works as expected and the requests complete successfully. What is causing the NOSRV? It does not seem to be the typical issue around no backends or redirecting or things discussed in other NOSRV posts.

haproxy -v

HA-Proxy version 2.1.2 2019/12/21 -

Status: stable branch - will stop receiving fixes around Q1 2021.

Known bugs: