I have several tasks on my workflow and made the Comment field mandatory for negative outcome. Before using Nintex form on the task, the it was working fine (using SharePoint stock task interface) but after applying Nintex form to the task (from the task ribbon), the mandatory comment validation rule stopped working. Now users are able to reject the task without providing Comment (although it is set as Required on the task). I also inserted the validation rules as suggested by Nintex Community. It did not work either. SharePoint stock task interface is working fine though.
Further investigation showed that the value of the Decision field on the task form is -1 (negative one) for both Approved and Rejected outcomes. Surprisingly, the workflow can distinguish between two different outcomes (although their value is the same).
It would be great if you can share your insight with me on this. Thank you!