As far as I know if you're a citizen of a country, you have to use your passport from that country to enter/leave that country, at least it's that way with USA:
[1] > U.S. nationals, including U.S. dual nationals, must use a U.S. passport to enter and leave the United States.
And the German foreign ministry says it's international practice to do so, but says "should" instead of "must":
[2] > Bei Doppelstaatern (Erwachsene und Kinder) ist zu beachten, dass nach internationaler Übung
> - die Einreise nach und die Ausreise aus Deutschland nur mit deutschem Reisepass oder Passersatz, z.B. Personalausweis
> - die Einreise in und die Ausreise aus dem anderen Staat (dessen Staatsangehörigkeit die Betreffenden ebenfalls besitzen) nur mit den nationalen Dokumenten des anderen Staates
> erfolgen sollte.
Also a requirement for Canadian dual citizens [3]
I guess it'd be interesting if one has a EU passport but is entering the EU through another Schengen country (eg. A dual Canadian - Italian citizen flying from Canada to Germany). Logic says keep it simple and show the EU passport, but is there a EU clause somewhere that requires that?
[1] https://travel.state.gov/content/travel/en/legal/travel-lega...
[2] https://www.auswaertiges-amt.de/de/reisepaesse-personalauswe...
[3] https://www.canada.ca/en/immigration-refugees-citizenship/se...