Websocket from one HAProxy backend to another backend fails

Question

I have two backends (b1, b2) sitting behind the same frontend (f1) in HAProxy, routing based on host name. I can establish websocket to both b1 and b2 from my laptop, however if I try to establish websocket to b2 from b1 (through f1) I get 1006 error (closed abnormaly - unexpected EOF). Pinging b2 from b1 succeeds. Here's my config:

frontend f1
    bind            192.168.1.1:443 name 192.168.1.1:443   ssl crt-list /var/etc/haproxy/ReverseProxyHTTP.crt_list  
    mode            http
    log         global
    option          httplog
    option          http-keep-alive
    option          forwardfor

    http-request set-header     X-Forwarded-Proto http if !https
    http-request set-header     X-Forwarded-Proto https if https
    timeout client      30000
    acl         b1  var(txn.txnhost) -m beg -i b1
    acl         b2  var(txn.txnhost) -m beg -i b2
    acl         hdr_connection_upgrade  hdr(Connection) -i upgrade
    acl         hdr_upgrade_websocket   hdr(Upgrade) -i websocket

    use_backend b1ws  if  b1 hdr_connection_upgrade hdr_upgrade_websocket 
    use_backend b1  if  b1 
    use_backend b2  if  b2
backend b1ws
    mode            http
    id          119
    log         global
    stick-table type ip size 50k expire 30m
    stick on src
    balance         source
    timeout connect     30000
    timeout server      30000
    retries         3
    server          s1 192.168.1.2:443 id 101 ssl  verify none resolvers globalresolvers 

backend b1
    mode            http
    id          117
    log         global
    stick-table type ip size 50k expire 30m
    stick on src
    balance         source
    timeout connect     30000
    timeout server      30000
    retries         3
    server          s1 192.168.1.2:443 id 101 ssl  verify none resolvers globalresolvers alpn h2,http/1.1 

backend b2
    mode            http
    id          120
    log         global
    stick-table type ip size 50k expire 30m
    stick on src
    balance         source
    timeout connect     30000
    timeout server      30000
    retries         3
    timeout tunnel 3600s
    server          s2 192.168.1.3:443 id 101 ssl  verify none resolvers globalresolvers 

Note: b1 and b1ws point to the same server & port tho b1 prefers http2 for grpc. If websocket header is present, b1ws will be used.