The sector is called "Tattoo Peak" in the first real campaign mission. I get no options. I originally wanted to put a colony there, but obviously it wouldn't let me. Now its directly adjacent to the colony, which is prepared for its first sector, but I can't annex it. When a colonizer is selected, the colony tooltip does NOT appear in Tattoo Peak. This is the only explored sector (that doesn't have something blocking the flagpole) that doesn't have the tooltip. I'm running 1.003.
Note how when fully zoomed out, the sector has a red hex, instead a yellow diamond, like the other sectors available to annex.
