Did some further experimenting. The artifacts were not visible in Gimp. I took the original vector file of one toadstool house and rendered it three times to png. One time on automatic in the target setting, second time I choose PNG and third time I choose Cairo-png. The first and second render had artifacts when I reimported them into synfig. The third one, the Cairo-png, did not have any artifacts. I also tested it on several different laptops and the brighter the screen the more clearly you could see it, hence the fact that Genete did not see anything in the first place.