Just out of coincidence, I experienced a similar issue a week or so ago. I happened to pair a bluetooth mouse with my Lumia 550, and Office (Word to be precise) started demanding I subscribe to Office 365 to be able to edit my document. Huh? My understanding was that Office was going to not require that for devices under 10", such as mobile phones. So imagine my surprise/ disappointment when that little yellow bar appeared telling me otherwise. Not to be discouraged I decided to unpair my mouse (a Logitech Ultraslim t630) and uninstall and reinstall Word from the mobile. I opened Word and that message didn't reappear. Then I (anxiously) paired my mouse again and... the O365 message did not reappear and I was able to continue editing my documents, the way I should have in the first place. A few days later Microsoft pushed some kind of mandatory update out to my phone again. When I went to edit my Word document, that O365 subscription message had come back. How frustrating. Again I uninstalled and reinstalled Word and that seemed to have fixed the problem.
Without understanding the cause of this issue, I suspect it's a bug. The workaround is to reinstall one of the Office apps, in this case it was Word. It's quite annoying because it's a 100mb download.
By the way, the Lumia 550 is my secondary phone that I use mainly for work purposes. My main/ personal phone is an iPhone 6 Plus. I like each platform for different reasons - but if I was forced to choose one over the other it would be the iPhone due to a superior selection of apps and accessories. Maybe MS could convince Apple to provide the option of live tiles and mouse support on an iPhone hahahaha.
Just my 2c.