Yeah, I use both, but not in that configuration. So haven't had the problem. That said, I only use the gmail app on my phone for gmail -- so the problem wouldn't appear. Microsoft have been trying to steer all users at any level to using an MS account for several years. 

I managed to get it back by trial-and-error switching from an Exchange account (which Outlook 365 surely is), to a personal IMAP account where I didn't have to manually set it up. So perhaps MS is trying to steer enterprise customers away from gmail.