Artboards sometimes export at 2x the size and sometimes half the size

I have an ongoing curious issue with XD. Say my artboard is sized 1000 x 1000px. When I export for web, I might get a set of pngs at 500x500@1x and 1000x1000@2x, OR on a different day using the same files I get another set sized 1000x1000@1x and 2000x2000@2x. It's driving me crazy as I can't work out why it behaves so unpredictably and it's causing workflow problems. Is anyone else experiencing this random behaviour?