Thanks @victordcq! I figured this looked like a bug, but don't have a web background--need to spend some time wrapping my mind around CSS.
We need dynamic rotation (using this in a view as template with rotation set in view parameter). I've already got it done with an icon overlaid, but am curious if there is a way to inject this transform directly in a property on the button so we can create it in an expression that takes the degrees from a parameter on the view? I tried a few variations under image/icon/style/backgroundImage, but didn't have any success.
I guess another way would be to create styles with various rotations and then swap between the styles--almost always want 0, 90, -90, or 180deg, which this solution would work fine for.