Manuss
Sleeping man
that tile was grasland in 836/837 (when Polynesia was introduced) and was changed into unpassable terrain in the subsequent patch because I discovered that it could be used to make early contact with the rest of the world
It is supposed to become grasland again after 800AD (havent checked yet).
So if you started your game with the initial patch that introduced Polynesia, saved your game, updated DoC and then continued your game, then yeah that grasland turned into ice for you
I took that pic after 800 AD.