Expanding on this after watching again.
Everything that Shion said in her graduation announcement:
Her heart hurt to announce it but she's graduating on April 26th due to a difference in direction with the company.
This is something she felt like doing for years and it's been building up. Her mental was at such a bad state before her hiatus last year, she was genuinely concerned for her life (basically implying she heavily considered suicide). Her mental health and physical health were both doing terrible and she had to visit the hospital multiple times. Her hiatus was meant to help with this, as she didn't want to end up hating this hobby. But even now, she still has to go to the hospital and it's likely that her mental will never be completely fixed.
However, one thing she's sure of after coming back from her hiatus is that she loves this. If this was her from long ago, she wouldn't have even gone to the hospital and would have just given up on trying to fix herself (again, heavy suicide implications). But because of all the love and support from the fans, she wanted to try harder at taking care of herself last year. She was able to think of the internet as a great place and she wanted to keep smiling with all the love from her fans. This is not a lie.
Of course, it's right to follow the guidelines and rules of a company you're a part of. But as the years passed, she had to do things that she didn't have to back then or things that she wasn't really ready for. This is where she thinks the gap between her and the company began. This is the main reason for her graduation. She was concerned about announcing it right before 6th Fes, but she wanted everyone to know that this was her final Fes.
She knows "difference in direction" has been said a ton, but that's really all this is and she apologizes that she can't say more. She plans to go into more detail at a later stream. There will be a ton of rumors and speculation around but please don't get tricked by that and just trust in her words. This is truly how it is, truly what she thinks and she hopes this won't be used maliciously out of context. There's still some time left so she wants to make a lot of memories with everyone. She'll do her best in 6th fes and she hopes we'll keep supporting her until the end.