I can identify two (mebbe three) things off the top of my head
1. She was on haitus
2. During that hiatus, her image has taken a hit.
3. (this one's iffy) she might have withdrawn a tad.
She went on haitus and as such she was absent for a comeback. As such fans were not able to see nor judge her performance in that comeback. Further: she was also absent for a lot of the performances as well.
This is not her fault but she did gain weight during her hiatus. This is not her fault because the hiatus she is not expected to maintain such a "idol's shape" especially when suffering from such a injury, surgery, and recovery. It is impossible, most say (and I agree). But there is one thing that she lost from that: her looks. It is much easier to like a person who is visually appealing and without her "visual-foot-in-the-door" it is much harder to vault back and win over the heart of the audience.
As is rumored, she also had bouts of depression and anxiety whilst on her hiatus. I don't know the specifics but what I do know is that I've had similar things, (for different reasons though) and when people say "It takes one to know one." It sometimes rings true. I don't know what specifically about her behavior is different but she does give off the general impression of being more reserved as a result and may not be risk-taking to win over the hearts of the audience. This last one is a textbook "Source: trust me bro"
