The conventional wisdom is that the switches go at the end of the field structure.

There is no mechanism whereby a field can revert to a previous description unless there is some document corruption, and this is true whether you use 2013 or any earlier version. The principles involved are the same.

There is clearly a problem with this document. I suspect that there is an orphan lock file involved. Close the document. Close Word. Check in Windows Task manager (CTRL+SHIFT+ESC) that the process Winword.exe is no longer running. If it is, close it from the dialog.

Reboot the PC.

See http://www.gmayor.com/what_to_do_when_word_crashes.htm with respect to orphan lock files.

Restart Word and open the merge document. From the mailings tab, set the document type to Normal Word Document and then re-attach the data source.

Does it behave normally now?

__________________
Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com