So, at the first episode of the second season, Master Fu said that two Miraculous were lost when the temple exploded: "the Butterfly Miraculous and the Peacock Miraculous."
But duing The Origins, Nooroo said that "The Moth Brooch gives someone their own superpowers [...]".
So, Hawk Moth's Miraculous is always going left and right between being the Butterfly and being the Moth.
And why is that! Iconsistency, my friends!
Okay, no. Maybe. Most likely. But to spare the american translators' jobs, I have another way to explain it!
So, you guys know that the Miraculous weilders' superhero forms' outfit and style are always changed to match the wielder's view of a superhero/supervillain. Thomas Astruc confirmed that and we can see that with all of the ancient chinese Miraculous holders and how their outfits are different.
Well, what if that happens with the...purple superpower-giving Miraculous? If the wielder sees themselves as a supervillain or just edgy, then the Miraculous can be refered to as the Moth Miraculous. This happens with Hawk Moth who sees themselves as a villain, and Nooroo, knowing that, refers to it as the Moth Brooch.
But if the wielder is someone who's...carefree and cheerfull, for example, the Miraculous is refered to as Butterfly Miraculous. This is why Master Fu refers to it as the Butterfly Miraculous. Since Miraculouses, especially the purple superpower-giving one, were made for the good (and the previous Moth holders were all using it for the good too), then Master Fu is used to it being referred to as being the Butterfly Miraculous.
Great theory! It makes a lot of sense too, as the Miraculous(es) were transferred through different cultures, their attire shifted to match. What you say here also states that the point of view of the villain would change the way it is described, or named. Nice job! In fact, that theory's so good, it just might be true!