parentPage.Xrm not working in html webresource from Outlook Client

We are upgrading from 2011 on premise to 2016 on premise

On a form we have a custom button that when clicked opens an  simple html web resource with a yes/no button. Upon yes or no we get some values from the parent form per example:

var oIncluded = parentPage.Xrm.Page.getAttribute("some attribute").getValue();

This works perfectly fine in IE/Chrome/Edge.

Just started testing using Outlook Client for 2016 and the same operation above using Outlook client does not work as parentPage returns null or undefined.
Is there a reason for this and a possible workaround?