Not sure if someone else already stumbled upon this:
When I have three images in a folder with the same name and suffixes of @1x, @2x and @3x and just drag the first one onto Xojo the other are loaded automatically…
But…as soon the image names have an additional dot in their name, like for example many SF Symbols have, it just loads the one you dragged onto Xojo. YOU have to drag the other two then manually.