It could be due to you running iOS12 since thats still in beta testing (public beta testing) and not officially released, probably later in the year (September ??)
Apple have only just released iOS 11.4.1 in the last week.
Word was updated a few days back to version 2.15.
I would say its not playing nice with your version of iOS.
They do say: From TechRader site
Both versions of the iOS 12 beta will be unfinished software, so we advise you to proceed with caution before deciding to install iOS 12 beta on your primary device. iOS 12 beta 1 was great for us, but we had minor reset problems with iOS 12 beta 2. Waiting for the final iOS 12 version is a safer choice if you rely on your iPhone daily.