SL3- Cannot edit text in scene view when using custom master slides
I've created a theme for my department based on the Harmony content library layouts, but with our color and logo on the master slides. However when I try to create slides using this theme in scene view, I'm unable to edit the text that is being pulled from the master view.
For example, my welcome slide in master view has a text box for "Your main course title". When I apply this layout to my slide, it has the "Your main course title" box but doesn't allow me to click in it to edit the title. I can edit the text in master view, but isn't part of the point of master slides to have text boxes that can be edited as needed in scene view? If I select a non-Harmony-based slide layout I can edit it just fine.
Am I building my theme wrong? Saving it wrong? Misunderstanding the purpose of master slides? I've attached my story and my theme, in case anybody is able to take a look.