Strange. Immediately after taking the above screen shot I used control-u to remove the FOW. That caused most of the rest of the map to show up, but the mountains just below and to the left of my sovereign, as well as some others at the end of the eastern valley, didn't - they stayed blank as in the above screenshot.
Reloading the game from an autosave, on the turn just before the one pictured above, and using control-U, revealed all the mountains.