Yup and just to be clear there is a properly functioning, recently replaced expansion tank on the boiler water loop.

Now I'm going to throw a sharkbite check valve on that domestic hot circ, between the circ pump and where it enters the cold line on the HW tank. What happens is when there is a large call for hot water (like bath or shower), the cold gets sucked in the reverse end of the hot water circ so you basically get hot/cold mixed at the faucet with only hot on.