Why does my dumpling /dimsum/momos turn dark brown while steaming?

We make dumplings by bun making machine. (bought from China.)

We add all purpose flour, xanthan gum and water to make the dough for dumpling.
We make 10 varieties of dumpling and filling varies as per the variety. After making the dumplings, we half steam and sell it to the market. When we half steam at our kitchen, everything if fine.

But some of our dumplings turns pale yellow when our client resteams at their sale point. If steamed more, it turns into dark brown colour.

What could be the problem?

Also, our dumplings breaks too much. What can be done?