this sounds to be a bug.

I've tried with other fields (due date, task name...) and it doesn't fire workflow either.

I'd suggest to open an incident with nintex support

From the sounds of it you use the Completion checkbox in the tasks list view.... Have you tested what happens if you actually open the task and complete the task that way?

Also, is the workflow setting "Require manage list rights" enabled while the user is not meeting the associated permission requirements (Manage List and Contribute) in order to run the workflow?

jpmhuls                      - Thanks for your reply. Correct, I use the Completion checkbox in the task view. Per your suggestion, I just tested to open the task and complete it that way, and the workflow still did not trigger. I do not have "Require manage lists right" enabled. Per emha's comment, I think I will get an incident ticket opened with Nintex.

emha                     Thanks for confirming. Glad to hear it is not just me. I will get a ticket opened with Nintex.