Ogata Risa confirms that it was indeed her private account and that everything written on it was done by herself.
Official statements, translated by Lurkette on H!O:
In mid-September an account on twitter (which was quickly deleted again) posted screenshots of an allegedly private instagram account of Ogata Risa. It showed her partying - which clashed with her idol persona she had shown so far - during Corona times, talking about boygroups, and saying she wants to quit Tsubaki Factory because she felt like management shoved her in the back. Some shots can be found online on matome sites.
The account apparently had selfies and other pictures of Risa that had not been published yet, giving it a feel of authencity. However, neither Up Front nor Risa herself reacted on the leak.
On September 30 new leaks were posted in which Risa badmouthed other members of the group, lamenting over Ono Mizuho's recent push, Onoda Saori allegedly having a new boyfriend and, maybe the worst of them, accusing Asakura Kiki of faking her hernia, which lead to her hiatus of nearly half a year in 2019, only to have a management member fired.
Tsubaki Factory had a release event on niconico scheduled that day, and Risa cried at the end of the performance. Some people linked her being emotional to the leaks.
/EDIT: This Japanese blog seems to have collected all screenshots in question: https://javelinmitsu.com/kokaratarisa/
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Not gonna lie, at this point I definitely thought it was fake. Can't blame the girl for venting, but damn, at least make it so that nobody can link it to you, like no selfies on the same account, not naming names, etc.
Like with Oota Haruka I can't see her coming back. She lost her coworker's trust.