True portrait mode
Currently, when portrait orientation is chosen, it affects only the direction in which the document scrolls. If page separation is enabled, the document is still oriented towards landscape.
Currently it is impossible to create a portrait document and print/export it in portrait mode as well. If someone were using Flow to write rather than draw (in portrait orientation), and then share the exported pdf, it will not look right.
Overall, it appears that portrait mode is second class in Flow. Note sure if it is being discouraged or the design is oriented towards landscape usage primarily.