I think at the age of the valve, the O-ring may be deteriorated which could allow the pink stuff into the water heater tank. That would be the only other way if the check valve is working properly, which is newly replaced so should be good. This should show the three way valve for your rig:
https://www.trekwood.com/products/14...-1-2-P-x-1-2-P
And this drawing I made years ago should be a similar diagram for your plumbing, at least the part where the water heater bypass valve is and works. And, if you didn't want to replace the three way valve, a simple on and off valve could be cut into the cold water line going into the water heater that would shut off water into the tank.