Even though fast mode leaves don't support transparency, their sides aren't actually culled by being next to other leaves or even opaque blocks, meaning their loss of transparency serves almostno benefit whatsoever.

This can be seen using spectator mode.

This is likely a regression caused by "fixing" another bug where leaves were transparent but caused culling.

EDIT: As it turns out, leaves DO cause fully opaque blocks (like logs) to be culled, but the internal faces of leaves are still being rendered (and opaque blocks don't seem to cull leaves). This means that somewhere (like jungles or huge forests) with lots of leaves does not really benefit from fast mode.

An easier test to show this is to use an ender pearl to teleport under a tree with a 1-block gap under it. As the attached image shows, you can see a culled log and into the void, but you can see internal leaf faces.