Add hasCompletedJoin() check as suggested

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -102,8 +102,11 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
             connection.getMinecraftConnection().close();
             return;
         }
+
+        if (connection.hasCompletedJoin()) {
             connection.getPlayer().getConnection().write(buf.retain());
         }
+    }
 
     @Override
     public void exception(Throwable throwable) {