I cant find much other than she Graduated High School (SSoPA) It was her health that caused the issue.
Channel-Korea has introduced Former Gugudan Member, Hyeyeon, Reveals The Reason She Left the Group
channel-korea.com
Gugudan has had Horrid health luck with members missing for all but 2 CB.
Now they are on an extended hiatus (ie the Not That Type comeback was November 2018)
Semina did not go well and Sejeong's solo under preformed.
I Can Not believe how every freeking agency except the one under hostile take over and crazy management has managed to screw up the IOI members groups.
WJSN does not get a pass with out the Rocket girls and that dynamic even with the added benefit of being senior to IOI as a group they were not setting the world on fire.
Given this history were I running any of the IZONE kids. I would move heaven and earth to keep the group permanent.
1 successful soloist and tier 2-3 groups is not a great freaking track record.