On my iPad Pro 12.9 (iOS 10.3.3) it often happens that I am writing Word document in Split View with Safari open as the secondary app. It often happens that I copy text from a website or the URL, and that when I try to paste it into Word, either nothing can be pasted (so there is no paste option in the pop-up menu) or it pastes whatever was previously on the clipboard.
I know it is a problem with Word, because when I open the Notes app and press paste, the text or URL selected most recently is pasted as expected. However, when I try to paste it from there into Word, often the same as mentioned above happens.
The work-around is to paste it in Notes, then copy it again in Notes and try to paste it into Word.
Word team: please try to reproduce this bug and fix it.