We do have an incredibly precise definition for up (RE'ers.) Given a polar coordinate system with the center of the earth as the origin, up is looking in a direction with increasing r. Of course, you are using Cartesian coordinates since you think the earth is flat, so I could use the same argument:
If up is up is up, then why do bubbles not always float up in a Flat Earth? With increasing r, we many times end up having up is up is left, or up is up is right. Therefore, since bubbles don't always float up in FET, it is very unscientific.