Anti Freeze getting into hot water heater

Stuck Check Valve. While it's down for the winter, change it out.

Agree with stuck check valve, had that happen to us.

OR just remove plug next time you get it ready to use......turn on water supply and let it blow out thru drain hole while you flush rest of plumbing system

Or just purchase on of these to flush the hot water tank:
https://www.amazon.com/Camco-11691-Water-Heater-Rinser/dp/B002XL2IBS

Yes your check valve is stuck open. I had the same thing happen. Getting the pink smell out of the hot water tank is now your concern. What I did, after changing the valve, was put an old piece of cut off garden hose onto the end of the hose, and feed it in through the plug or anode rod hole, then spray and spray, dump and dump...

1 valve system would be a 3-way valve
Turn handle parallel with cold water line and water goes into WH Tank

Turn handle 90* so it is perpendicular with cold water line and water 'bypasses' WH tank. Goes to hot water line and then hot water plumbing. Check valve in Hot outlet stops back-flow into WH Tank


2 valve system.......
Have to CLOSE cold water inlet to WH Tank
Have to OPEN bypass valve
Check valve on hot outlet stops back-flow into WH Tank


Problem...
Incorrect valving
Bad valve
Bad check valve

Disconnect cole water inlet at water heater to check for bad valve

If valving correct then most likely bad check valve in hot out

Sounds like you either have the bypass valve in the wrong position, or, the valve has failed. Easy to replace and relatively cheap. But first, take it out of the system, making sure it is in what you believe is bypass position and look.