Actually, this is an expected and documented behavior. Read the first few chapters here:
construct.net/en/make-games/manuals/construct-3/plugin-reference/drawing-canvas
DrawingCanvas's snapshot resolution changes with window size, but only after you draw something on the canvas. If you paste objects and then resize the canvas, the snapshot resolution doesn't change. And if you draw even one pixel on the canvas after resizing (to update snapshot resolution), everything else on the canvas is cleared, so all your pasted objects are lost..
In conclusion - it's a giant PITA to get the correct size of exported snapshot!
You can probably resize the canvas in advance (for the target snapshot resolution), then scale layers with object to fit on this canvas, then paste, then export. But I haven't tried it, it's too much work.