Yeah, tvtropes calls it Instant Cosplay Surprise. It's one of those odd artifacts of visual media, like how nobody can see somebody standing just out of frame, or how manga and comics can have people reel off whole paragraphs of speech between punches in a fight. Nothing actually exists between panels, so changes to a character's appearance don't occur until the new frame shows them in a new outfit.
As a side note, I'm starting to think that all we need for multilateral nuclear disarmament is for Kotori to look at the relevant world leaders and ask, "Please?"