Unexplorable regions?

[SonOfAGhost]

Has anyone else encountered a region elsewhere on the map that they don't get the exploration option?
SW corner of Egypt, region ID # 484

I set up a supply depot on the Nile just South of there in Uweinat to support a defensive force protecting 2 explorer parties. From there I've successfully explored Kufra, to the West, and have had the other party working on region #558 to the South. However, #484 to the North never goes green when I select exploration mission.

At this point, I'm hoping that whatever is hidden there will explain why the Egyptians refuse to surrender despite a warscore over 300 and total occupation for more than a decade...

[Offworlder]

Is that the region where the Mahdi emerges? I believe that only comes to light when the Mahdi events starts.