I have a Nintex Form for the properties of a document content type. 

Documents are created by a workflow with some properties populated in that process.

The document template contains Quick Parts that are connected to some document properties.

Fairly consistently...an item can be managed using the properties form, UNTIL the document is first edited in Word (there are some free text areas in the document that are intended to be edited that way).

I say fairly consistently as it sometimes seems to take a couple of goes to 'break' the Nintex Form, but generally, after editing with Word, when next trying to open the properties form we get the error:

The encoding in the declaration 'utf-8' does not match the encoding of the document 'utf-16LE'

Can anyone suggest what is happening here? 

I've had a look at playing with Word settings below but I don't think they're really relevant: