Continuity mistake: When Joe and Iris are going to talk to Matthew Kim, Iris is already wearing the suit that Cisco later makes for her, she just wears it open the first time and closed the second time. (00:13:41 - 00:29:50)
Suggested correction: She is wearing the jacket when speaking with Matthew Kim because it is her jacket. When Cisco has her new outfit in the suit bag, he said something to the effect of "I had to make do," meaning that it was short notice and he could not make her a suit like he did for other speedsters. She is essentially wearing regular clothes with a mask, and her jacket was part of that outfit. Quite frankly, all the clothes could have been hers, but we know the jacket is.
Not only that, but in the scene before, we see the jacket hanging over her chair. So just something she wore that day.