Does anyone have the serial number range for MCX’s being able to function reliably with the standard AR trigger?

I don't think they ever 'really' did. My understanding is that the MCX is particularly hard on triggers. In fact, everyone offering an MCX specific Triggers does so due to the issues associated with trying to use a std. AR Trigger in one.

Gen 2 added the firing pin safety which needs a hammer that can hit the FPS lever. There was a different FPS lever that would work with Geissele triggers - the "G" lever.

Sigs hammers work, Geissele's MCX trigger works, and various others do as well. Timney's two-stage DH3, Trigger Tech has an MCX trigger too.

Or you can remove the FPS and use any AR trigger.