How to select email template for Opportunity?

[rwhit]

I have created an email template for the Opportunity entity. If I create an email with a To of either Contact or Referral for the Opportunity and Regarding as the Opportunity, the template is not presented - only Contact or Referral (respectively) templates are available. According to the documentation (https://docs.microsoft.com/en-us/powerapps/user/insert-email-template), if both To and Regarding are specified, a dialog should be presented allowing the user to select which type of template to use. 

Am I missing some configuration?

Note: if I only set the Regarding field, I get this error when I click Insert Template: "Select a recipient or a regarding record that can be associated to an email template. For example, you can’t select a queue."