The bad cartridge (causngcausing backflow from cold into the hot line) might well be in a different valve. There might also be more than one.
Look for a faucet/fixture where the hot line (run some hot to warm it up before turning the shower on) gets cold when you run the shower, and change the cartridge there.
Or, use the shutoff valves that should be present at other fixtures to temporarily turn them off to prevent backflow mechanically, and see if the problem goes away, then see which one brings the problem back.