I'm using both Apeks and Poseidon regulators for EAN40. These are not O2-certified nitrox regulators. You are not using 100% oxygen, are you?
Nitrox regulators exist for three reasons:
1) the regulating body wants different ports (connectors, thread sizes) for different gases as this makes it more difficult to plug pure oxygen on a non-oxygen clean high pressure system (1st stage, cylinder). Such a mistake could lead to one of the brighter moments of your life. People have used non-O2 certified equipment with oxygen for decades, but then they also oxygen cleaned them, maintained them well, and made sure the o-rings and other stuff are compliant.
2) somebody might sue the regulating agency or its employees, so it is best to just deny everything
3) it's great for marketing to sell special gear for each gas