hot water in cold line

Often, thermal expansion will push hot water out of the tank, into the cold supply pipe. Then, drawing water from the cold pipe for a fixture will introduce that heated water. Most noticeable if the water heater only has a short drop off the main Cold line.

There are special nipples to be installed on the heater inlet/outlet. They are called "Heat Trap" nipples and have a small ball check restrictor built in. Maybe the old tank had them.

Jim

Do you have a recirc pump that keeps hot water available for immediate use at faucets far from the heater (upstairs, etc.)? If so it could be pushing hot water through a leaky single stem faucet.