Unable to use variables in 'Assign Task' Workflow step email template

I have a form Workflow that maps some variables from a form submission, sends an email with a custom template that references some of those variables, then triggers an 'Assign Task' workflow step.  The 'Send Email' step works perfectly, it includes the data form the form submission in the template as expected (via something like ${variableName} in the template markup). However, if I do the same thing in the 'Assign Task' workflow step, the same variable does not work. It just gets printed in the email as plain text.  This is despite other default Workflow variables in the same email template working correctly (ex. ${workitem_title}).

Does the 'Assign Task' workflow step just not support variables in email templates, or am I doing something wrong?  See attached screenshot of template for example.  Thank you!