I have a custom button called "Sync Task" added to my Dynamics 365 app using the classic and new unified interface. The custom button is suppose to open a Web Resource HTML page in a Modal Dialog when clicked... but instead opens it in a new tab.
Classic Interface Button
Unified Interface Button
It opens the page in a new tab at the Web Resource location instead. Not sure what I could be doing wrong.
https://game.crm.dynamics.com//WebResources/SyncTaskLoadingPage
My Dynamics Environment
Dynamics 365, version 9.0
My Ribbon Workbench Environment
Ribbon Workbench 2016
Verion: 3.1.177.1
Author: Scott Durow
Button configuration
Action configuration, notice my Windows Mode is set to "ModalDialog".