Photoshop CC 2018: Exporting @2X worked fine until the latest update. When viewing @2X exports on a Retina screen (or 4K), they look blurry, like normal @1x. Not doing anything differently, nothing else changed expect for Photoshop updating to CC 2018.
That actually worked. I did a quick test: open an 800x800 JPG > Export as... > set the resample selection to "Preserve Details", and the @2X version of the export is sharp and crisp on a 4K screen. Many thanks for this tip, it is very much appreciated.