Yeah, that's a bug. If you want you can edit your save file
C:\Users\*username*\AppData\LocalLow\Glitch Pitch\Idol Manager\data\manual_saves
Find "data_girls__Girls", find your idol in there and change "status": 6 to "status": 0
Just a note, I was having the same problem and this worked great for me, but I also had to edit the auto_save file to save my hiatus'd idol--I think because I used "save and quit" when I quit out of the game before editing the save files. So, yeah, if anyone tries this and it doesn't work, I'm guessing editing both the manual AND auto save will do the trick.