Issue
When viewing the task form in the mobile browser view (Safari or Chrome) the SharePoint Task List form is displayed instead of the Nintex Task Form. Clicking on Cancel will then load the custom Nintex Task Form.
This occurs on iOS 12.2 and Android 8.1.0 and in Nintex Workflow 2016 version 5.0.1.0, Nintex Workflow 2013 version 3.4.0.0.
Resolution
The workaround is to set the Nintex Task Form content type as the default content type of the Workflow Task list or to disable the Mobile Browser view setting on the list/library.