I'm unable to set the mode for a parent frame

I’ve gone through the variables playground docs and everything seems straightforward, but I cannot assign a mode to a frame. When I look at the example docs, there is a mode switch icon in the layer palette. But when I draw a frame on the canvas, I don’t see it.

I’ve placed a demo file here. I want the red box to change its red variable between light and dark frames.

Hey Paul, I looked at your file and everything is fine. You can change the variable mode of any frame as well as the page.

This is weird. I went back to the file today and see that the mode switch icon IS present now. Maybe this is a bug? Or did somebody change something?

When I add a frame and apply a variable color fill, the mode switch appears. Yesterday it did not do that.