Add the same check to ClientPlaySessionHandler

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -122,13 +122,17 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         }
 
         // If we don't want to handle this packet, just forward it on.
+        if (player.getConnectedServer().hasCompletedJoin()) {
             player.getConnectedServer().getMinecraftConnection().write(packet);
         }
+    }
 
     @Override
     public void handleUnknown(ByteBuf buf) {
+        if (player.getConnectedServer().hasCompletedJoin()) {
             player.getConnectedServer().getMinecraftConnection().write(buf.retain());
         }
+    }
 
     @Override
     public void disconnected() {