We found the cause of this problem. There were several worksheets in the workbook we were exporting to. The other worksheets had styles and formulas in them. Some of the cells with not marked as locked. When Monarch exports to a workbook with cells that are not locked, cell styles that are not locked and all subsequent cell styles are deleted. Since the styles were missing the file didn't open correctly.
The work around is to make sure all cells are marked locked.