If a TOC field uses TC fields only (with the \f switch) and there is a tab in the text of the TC field, after mail merge, the tab becomes really wide. If the file is opened in word and the field is updated, everything goes back to normal.

Expected:

Actual:

A possible workaround is to set the fields to be updated on opening of the document:

DocxFormatProvider provider = new DocxFormatProvider();
provider.ExportSettings.AutoUpdateFields= true;