We are having the same issue, but when exporting the workflow and importing it into our QA environment for testing.
I am seeing this happen when there are instances of the workflow running. Once those stop and new tasks are generated, the form changes appear.
i have the same problem. it is save to deactivate Nintex Forms for Workflow feature and then to activate again?
I had no custom form showing up at all and found this thread.
To whoever is wondering : Yes it is safe
There is scary warning but disabling and re-activating right away fixed my problem. (even on tasks already created)
I did it on site level.
http://FARM/SITE/_layouts/15/ManageFeatures.aspx?Scope=Site