On the Usea page I didn't find the information I was looking for (please feel free to correct / comment on my observations):
1) The original AC 4 maps provided no states and/or nation borders on the continent of Usea, neither did the upgraded official Strangereal world map released for AC 5
2) The first Strangereal map illustrating Usean state and/or nation borders arrived with Ace Combat: Assault Horizon Legacy in 2011 and illustrated Usean state and/or nation borders prior to the Ulysses Impact Event, the Continental War etc. (and is apparently the source for many fan art works that feature strangereal maps with those borders on the continent of Usea)
3) The official AC 7 Strangereal map released in 2018 (with the coordinate system) and the one illustrated in Aces At War: A History 2019 feature the same border lines as the 2011 version but with the new separation of the Gunther Peninsula region from Erusea.
Is this summary correct? (please feel free to add additional observations)
The Strangereal map used in Legacy is not prior to Ulysses. It's after. They simply copied over the AC5 map, because you can see all the crater names. Yes, it doesn't make sense since the game takes place before Ulysses, but that's how it happened.
And yes, the only border difference between the Legacy map and the 7 map, on Usea is at Gunther.