protobuf: Nested composite fields sometimes not being marshaled

syntax = "proto3";

package package;
option go_package = "proto.definitions/generated/package";

message Point {
	sint32 x = 1;
	sint32 y = 2;
}

message PointCommand {
	Point src = 2;
}

message PointEvent {
	Point src = 3;
}

package main

import (
	"fmt"
	"io"
	"net"

	"google.golang.org/protobuf/proto"

	pd "proto.definitions/generated/package"
)

var watchers map[uint64]chan<- proto.Message

type Session struct {
	writer io.Writer
}

func (sess *Session) Listen(conn net.Conn) {
	sess.writer = conn

	for {
		// data []byte : read from conn

		var cmd pd.PointCommand
		if err := proto.Unmarshal(data, &cmd); err != nil {
			panic(err)
		}

		if cmd.Src == nil {
			panic(fmt.Errorf("nil src"))
		}

		evt := pd.PointEvent{
			Src: cmd.Src,
		}

		for _, w := range watchers {
			w <- &evt
		}
	}
}

func (sess *Session) Watch(ch <-chan proto.Message) {
	for msg := range ch {
		data, err := proto.Marshal(msg)
		if err != nil {
			panic(err)
		}

		// send data to sess.writer
	}
}

func ListenAndServe(addr string) error {
	l, _ := net.Listen("tcp", addr)
	defer l.Close()

	for {
		conn, _ := l.Accept()
		sess := new(Session)

		{
			var id uint64 // generate unique id
			ch := make(chan proto.Message)
			watchers[id] = ch
			go sess.Watch(ch)
		}

		go sess.Listen(conn)
	}
}

func main() {
	go ListenAndServe(":xxxx")
	<-make(chan struct{})
}