@Simeon - not a bug, just an observation. I have noticed, in editing mode, that switching from portrait to landscape sometimes seems to operate smoothly in the turn. But sometimes the turn gives an editing window to the left which I assume is 768 pixels (my iPad res) with a black section to the left which is then removed. It just looks a bit messy. Other times the rotation is pretty smooth. If you changed to a white background before turning we wouldn’t notice it.
@Simeon - I have a project designed to run in landscape mode, but ran it in portrait - then rotated to landscape. Background width was retained so there was a coloured section on the right instead of the image. I had forgotten to check for aspect. Remenicent of the editor observation above suggests your check for aspect is delayed.