While I was working with a client, we stumbled across these issues. They have this request to add/modify an existing SharePoint designer workflow task form with new fields. This can be a simple change, but I came across these two issues. Here is how I solved them.
Issue: Field Update
Update the form fields (As required):
No reflection in InfoPath form.
Even after force updating from Custom Task (CT), there was still no success.
To Fix Field updates:
Remove/rename/delete away the form from workflow:
Re-updating the Custom Task from Workflow will generate the new form and fix the issue.
Issue: Custom Task Unable to show up in the task form
While updating the task form for the user collection activity, you will see that the Custom Task is updated with no custom fields.
To Fix a Custom Task not showing in the form:
Proceed with adding back fields. Here you need to know previous fields, so you can track back from previous task forms.
Delete away the existing InfoPath form for that Custom Task.
Publish the workflow. This will fix the Custom Task and form.