Yes, she's Mai, and we don't know how that happened, we can't just say it's just another plothole because we don't know nothing about it yet.
And I had a theory, that personally seems too farfetched and kinda ridiculous, and probably it wouldn't matter to tell it because we'll have lots of answers in like just 3 days, but whatever.
After apparently defeating all menaces to the timelines, Trunks went back to his Earth from his job as a Time Patroller and started helping the rebuild of the civilization.
But, remember how Miira's objective was to get as much of Goku's DNA as possible in DBO? Maybe he finally did it, and got alive, and decided to transform himself into Goku, and now, with an incredible power available, he managed to get his way to become a God of the Demons, hence the earings, and now he is getting revenge on Trunks, and destroying all humanity, because they were the ones that defeated him (+Majins and Nameks, but you get what I mean), and he wants this to repeat in all timelines, to become timeless or something like that.
And this would also explain what Akira Toriyama meant when he said white hair would've been confusing against the next villain, since he's still Miira, his SSJ hair will be white, or maybe he will reveal his true form as Miira later in the series, who knows.
This would be a continuation to whatever happened in DBO, or Xenoverse if Toriyama wants to put that mediocre story in the canon.