Using a background color with some alpha (e.g. #99fe0038) when the shadow attribute value is set to true reveals two circles: one for the floating action button and another inside (cf. image below). The latter is probably caused by the shape of the drawable that is used for creating the shadow.

It would be nice to get a circle without the inner one when a transparent background color is used.

I have also noticed that the image drawable used for the shadow isn't provided for xxxhdpi.