Hello RMMedia,
There was an update, the customization for the FSlateBrush to automatically keep the margins set correctly. At most they can allocate 0.5 across all values, if left is 1, right must be 0, if top is 1, bottom must be 0. If you attempt to set them all at once, by setting margin to just 1, it will fallback to 0.5 or all of them. I hope that this information helps.
Make it a great day