JFC, you do she know has a N2, the 2nd highest proficiency level, in JLPT? 
You misunderstood what I was saying. My previous post was not about whether or not she still "knew" how to speak Japanese, but about how it sounded to me in that video like she was speaking/singing it with a bit more of an accent, like how she did when she first joined MM. By the time she and Linlin graduated from the group, Junjun's verbal Japanese was really quite "smooth". Yes she still had an accent, but it wasn't as pronounced as it was in the video on which I was commenting. In all likelihood this is just due to her not having much "need" to speak Japanese now that she's moved back to China.
It's been around a year since Junjun has lived full-time in Japan or been surrounded by people who speak nothing but Japanese. And it's not that likely that there a lot of Japanese people living in China that Junjun can converse with in Japanese. You can have all the "book-smarts" about any language in the world, but as anyone who has studied a foreign language will tell you, the key to staying
fluent/proficient in it's actual use as a spoken language is having the opportunity to actually practice/use said language on a constant, regular basis. The more one speaks any foreign language on a regular basis, the easier it becomes for them and in many cases, the less noticeable their accent becomes.