WORKAROUND
To work around this problem, use one of the following methods:
Method 1: Change the Main Document Mergefield
Remove the word "MERGEFIELD" from the merge fields in the main document.
To do this, display field codes by pressing ALT+F9. Delete the word
"MERGEFIELD" from within the field code.
Ignore the following error message and perform the mail merge:
Error! Bookmark not defined.
Method 2: Use Equation Editor
To maintain superscript or subscript formatting, use the Equation Editor
to create the superscript or subscript characters in the data source.
Method 3: Use a REF Field
For the data field that contains special character formatting, insert a
REF field in your main document instead of a MERGEFIELD field. For
example, in place of the following MERGEFIELD field
insert the following REF field:
Ignore the following error message and perform the mail merge:
Error! Reference source not found.
Word retains the data document formatting for the results of the REF
field, which preserves the special character formatting. The drawback to
this workaround is that the Mail Merge Helper does not detect errors or
suppress blank lines for this field. If you use this workaround, you must
suppress blank lines and resolve any mail merge errors manually for the
REF field.