Button Transparency Broken

Had same problem. Turned out that the theme applied a backgroundcolor to buttons as well. 
If you check the Button under the "General" option and then "Effects" -> "Background" it might be turned on there. 

Thanks for the reply.  I don't think I would have been able to resolve this without the pointer.

You were correct, the theme was the culprit.  I'm using the default theme, so, to fix it, I went to "View" > Click the theme dropdown arrow next to the theme icons > Select "Customize current theme."  Power BI then prompted me that Microsoft had made updates to the default theme and provided an option to update.  I chose the option to update and the button transparency works again.

Thanks again!