I'm not using Nintex Forms. I have a custom Content Type attached to a Request data action that has a people picker control and it looks and functions differently than the OOTB 2013 client-side people picker control. How can I force the standard OOTB people picker control on workflow task forms?
For the request data action in the workflow, you should still be able to format that task form using Nintex forms which may help with the expected functionality you are looking for. Can you provide a screenshot to help us better understand what you are seeing?
the reason is that task form is still generated by nintex and it uses its own people picker control and not sharepoint's one.
I affraid you can not much to do to change it.