I have the same issue. We upgraded to the 11.9 version of Words and we are getting this exception on code that has been running fine for the past year.
here is the image of the exception -Unexpected end of children nodes reached.
We use a custom iFieldMergingCallback as well.
The methods called in the iFieldMergingCallingback include Tablename, isEOF. I don’t see the GetValue being hit.
Is this a new bug?