Proxied gRPC gives occasional 502 error

I’ve checked on both production and staging server, and a proxied gRPC services occasionally returns a 502 error. If I use direct IP, it never happens. Example of a “Ping” loop with an error:

➜  ~ while true; do grpcurl $HOST ping.Ping/Ping; done
{
  "message": "Hello !"
}
{
  "message": "Hello !"
}
Error invoking method "ping.Ping/Ping": failed to query for service descriptor "ping.Ping": server does not support the reflection API
{
  "message": "Hello !"
}

I’ve disabled proxied mode before I get to the bottom of this. What’s the best way to debug?