Leaves do not cull on fast mode

Of course this shouldn't be the case with fancy graphics, but when graphics are on fast mode, I would have expected leaves to cull with other blocks, including themselves. This doesn't seem to be the case as far as I can see using spectator mode and going inside a leaf block/trees.

The leaves' parent model has cullfaces enabled, so it doesn't seem to be an issue with the model itself, but rather something to do with the game.