Do the compass roses on the hidden map mean anything?

It's purely cosmetic, in order to look like an old map as we usually represent them.
I know this is a rather short answer, but there's nothing more to say, really.

I believe they are cosmetic.

From memory, in the lead up to the release of Civ VI the developers mentioned that they changed the Fog of War to look like an ancient map, rather than the White Clouds of V, or the Black from earlier games. The developers have never mentioned anything about the Fog Of War having changed it's purpose in Civ VI.

The purpose of the Fog of War in Civilization games has always been to obscure the unexplored sections of the map. It would go against this purpose for that Fog to give you clues about what you have not found, and such a big change would have been mentioned.

Finally, in my 700+ hours of playing Civ VI I have never noticed any patterns, or indication that these map pictures mean anything at all. As you said, you can't find any mention of it online, and I am sure that across untold thousands of hours played by the player base someone would have noticed if there was a pattern.