yup. looking at the code it says the throbber should be centered. and mines at the left side.
SNew(SHorizontalBox)
+ SHorizontalBox::Slot()
.Padding(FMargin(25, 0.0f, 0, 0))
.VAlign(VAlign_Center)
.AutoWidth()
SNew(SCircularThrobber)
// Convert font size to pixels, pixel_size = point_size * resolution / 72, then half it to get radius
.Radius((InScreenDescription.LoadingFont.Size * 96.0f/72.0f) / 2.0f)
]