For some reason, I'm not getting the correct display in the fax field when I go to Sales > Orders > Order Details. See attached screenshot. Which template should I restore to fix this problem? Or is the problem being caused by something else?
You need to check here:
admin/controller/pages/sale/order.php
or
admin/view/default/template/pages/sale/order_details.tpl
Look like there is not closed tag or broken HTML