First of all I must declare that I have an assumption in this study: Australia exists

In the fantasy round earth, Australia is roughly opposite Europe. Aussies are upside down compared to us in the north. This is clearly absurd, so to get around it, roundies say that Australians are used to being the wrong way up, so they don't notice! Okay, I'll go with that, but what about European visitors to Australia? Surely they would notice, but nobody ever reports that they flipped upside down at any point in the journey to Aus. Of course they don't, we are all on the same plain.