I don't think the knee thing is fake. It's just how the producer staged the whole things. Had Yuhane's injury been mentioned at the start, it would be explicitly perceived that the producer favors her. It was similar to how Momona was not featured in almost every episode of PD101, except the last one. However, based on Yuha's reaction when she was announcing that she quitted, I deduce that her injury is an exaggeration. Furthermore, if her injury is worse after joining the group for a few months, then the company can be blamed and suffers more financial loss.
Also side comment, in the Mikitty interview with Ayumin, she said something along the lines of "after working on an audition tv show, I realized how skilled everyone in hello project is" lmao. Maybe that's what Yuha realized too!
This is worth highlighting. I don't think it applies to current MM members, and the problem of H!P right now is that the members don't deliver the performance that reflects how skilled they are like Mikitty mentioned. Also, H!P company tries too much to minimize interactions between H!P and non-H!P idols. As a result, I think if you ask idols who were born in 80s and 90s about Mikitty or Michishige Sayumi, they know. However, if you ask idols who were born in 2000s about any current MM members, they will give you question marks.
