Really weird… I just had a similar thing happen to me on my laptop. Windows 10 latest, Office Pro 2016, and it just started not typing characters reliably. Here is me typing “This is a test” 4 times:
hs i att
hs s est
Similar results with another Keyman keyboard, and when I turned the Keyman keyboard off, everything returned back to normal. But the really weird thing is that that happened ONLY IN WORD! I opened Excel, LibreOffice Writer, Outlook email (which is supposed to use Word as an editor), and even typing this reply - and in all of these the Keyman keyboard worked just fine.
So then I closed Word, and re-opened it, and now everything is back to normal. So I don’t know how it got in that state, but restarting Word seemed to fix it. So Emily, is this still a problem after restarting Word?