I can confirm that this does happen with gifs.
What seems to be happening is the properties for all actions of the button are being changed when using an animated gif.
i.e if you set the fill to an animated gif on normal, and then go to Hover and set it to none then go back and check normal, you will see that that is now none also. This happens in the reverse order as described above also.
This does not happen for normal images.
One work around for now would be to use a css style in the head and give the button the class or ID of the style.