So, this happened today:
Made a band including Joel, Natasha and Collin. Saved the game on default screen (with the playlist, and the list of buttons such as Manage Band, Band Meeting etc). Updated the game to the latest release today. Loaded saved file. Game went back to the Modify Band Screen with all characters exiting the band except Robin. Also I received an warning for a split second saying that Owen left the band. (?)
Possible update hiccup? I'll try a new game just to check.
PS: I had already finished a few events with Collin prior to saving.