I think I might have seen this issue mentioned before, but I can't find it on the forums.

Ａnyway, I'm playing the main campaign, and I'm still in the early stages. While dealing with Olooson and Heracleum had rebelled and were now under my control. At this point, I still hadn't taken Methone and Pydna, and I hadn't unlocked the "In the Shadow of Mount Olympus" objective. Sadly, it now turns out that the objective remains undiscovered, even after taking Pydna and Methone (and tearing down their walls）， as well as the nearby shrine. Is there any way to get the objective to unlock, or am I just doomed to restart?

Thanks for responding. I couldn't figure out how to bring up the console, so I just started a new game. No big deal, since I was still in the early game.

From what I can tell, those regions where discovered when Dion and Heracleum rebelled and reverted to Macedonian cities, but since I didn't move any units into those regions, the discovery didn't trigger the objective. When I later moved units in there, the objective didn't trigger, because the regions where already discovered. That's my theory, at least. Let me know if you want the save. :)