Placing Leaves(maybe any transparent block) on chiseled blocks makes this render bug.
Reproduced in Minecraft forge 12.16.0.1797 [1.9] with Chisels and Bits[9.5] no other mods, no Optifine, only forge and C&B.
Thanks.

It seam that you "fix it". I thing i know how you "fix it" and why that was happening most likely he was in Fancy block rendering of leave as he most likely did not try others transparent blocks or you made it with fancy block rendering in mind. The "fix" I was that now the bit render in Fast. If that is the case not to me as I have to play my modded world in Fast but maybe those who use it in Fancy and like how the leave look might not like it.

PS: mostly speculation.

This has to do with how C&B does bit surface erasure. Which normally works fine, since all glasses for instance render as "Transparent" any transparent surfaces will block other transparent surfaces.

It breaks some with the cutout layer, which is used for Normal glass, Leaves, and Grass oddly enough.

I'll probably just need to make any separate cutout material behave separately.

Fixed by 3404c83 !