Padding and HorizontalContentAlignment are not properly applied when Image is used.
Available in minor release 2018.3.1109. It will also be available in the R1 2019 release.
If you have set the HorizontalContentAlignment to some other value (End, Start) and you run the application in Release mode in UWP -> the value is reset to center.
In the cases when a partially transparent image needs to be used so that the BackgroundColor of the button should be visible in the transparent areas of the image. Currently the image completely overrides the color. We should consider allowing the use of the two properties alongside.