I’ll bet anything in the world that either your recirc lines didn’t have a check valve, or if they did, it was a swing check valve.
My friend Larry Weingarten has long advocated the use of spring check valves on recircs, but he’s the only one I know who has ever actually used one.
Swing valves can get stuck open in hard water, and then they don’t do anything at all. Then cold water — and sludge — can be drawn backward into the recirc line during periods of high water use.
You might like to go to the commercial water heater section of my site and look at the recirc valving diagram from Larry’s Water Heater Workbook.