If you use any ramtester software you test one combination under one environment. Maybe the counterfeit chips don't allow as long traces. Their setup or hold ti

If you use any ramtester software you test one combination under one environment. Maybe the counterfeit chips don't allow as long traces. Their setup or hold time are slightly different which might be an intermittent error. Or only an error when in an 50 degree celsius+ environment. It's trivial to test RAM under a single condition like you are proposing. It's almost impossible to check whether RAM performs as expected over all scenarios.