I am having the same problem.
My sheet is linked to a Form. Some of the answers are optional so do not populate the cells in the Sheet.
In the first part of the merged document, the field codes (for additional stories or poems) do not show and all I get is a blank space, which is fine.
The last two question on the Form, which are the last two columns in the Sheet, show the field code if they are not populating.
I have tried rewriting the template from scratch and changing the headers but nothing seems to have any effect on this irritating glitch.
Any advice welcome.