Was it in a parallel universe I used to have an option to create a document with a transparent background? In my parallel universe transparent background worked yesterday.
You don't need to click on Advanced Options, just scroll down.
When you click on the drop down, to show the background options, they get pushed downwards, off screen and behind the Close/Create buttons. Clicking on Advanced Options does the same thing, pushing the screen down, but with an unnecessary step.
This seems like a bit of an oversight by Adobe's design team, so hopefully they fix this in a future update.