Some face types will randomly cease displaying their texture and will remain this way until the game is restarted, which results in other face types exhibiting the problem which did not before it was restarted.
It is worth noting that not only have I never seen this bug before in S.O.G., but I don't think I've ever seen it in any Arma game. Sometimes when loading a mission in the editor faces may appear this way as the game loads textures in, but they never remain stuck like this indefinitely.