For some reason I've been quite lucky that only Vtuber I didn't like too much graduate.
I wouldn't say that I did expect Sana to graduate, but there have been many hints that she wasn't fully happy (Kiara hinted it at that some people might join Hololive not being aware how much work it is and how strict the Japanese work culture is once, but didn't say it was Sana, but I always thought she was talking about her going by context) and that she also suffered health issues (back pain), so it's not really a surprise to me.
Due to her streaming so little, she barely got followers, I think she has the lowest subscriber count of all Hololive. And she basically broke the company's contract to upload at least 3 videos per week all the time.
On a sidenote, back pain sucks, if you succumb to it, you can basically not do any jobs anymore because it's either too much sitting, too much standing or too much lifting. I always had to quit my job because of it, but ironically COVID-19 saved me by introducing home office, allowing me to lie down and go for walks frequently (strangely even though I'm only actively working at the computer like 5 hours per day at home office, my productivity has actually increased, that's why no one is complaining).