Here's a good one. In the latest release of Nintex Forms for SharePoint 2016 (126.96.36.199), there is a bug in the Person or Group field control.
If you have a person or group field on a list item in SharePoint, that allows groups, and the list item field is pre-populated with a group name, if you have that field on a Nintex Form, it will empty the field contents on submit if the field has a group name in it, and you don't alter the content of the field while in the form.
We had this occur in a group of sizeable workflows - it took 4 days to narrow it down. I have informed Nintex support.