It looks like this is a part of more complex issue with SVG. E.g. "Cloud" engine can't mirror, but can invert, and "Local" engine can't invert but can mirror, and yes, there are issues with copper area time to time. I found temporary workaround (I use chrome) - it can do as you need if you close all tabs inside easyEDA browser tab, then close entire tab. You can leave user-center tab opened and open your project from it in a new tab. Then, you can try to export as SVG with necessary params again. But you need to verify as that time export, as further exports with different params and repeat close/open if something went wrong.