The now-solid faces of stairs and slabs (specifically the bottom face of stairs and slabs in the default orientation) now block chests from opening if placed above the chest. This does not happen in vanilla. Non-solid faces do not have this effect.
Slabs and stairs also transmit light incorrectly on the opposite side of their models, which does not happen in vanilla.
A tube made from stairs and solid blocks: https://kek.gg/i/5NjLBg.png
Correct lighting in vanilla: https://kek.gg/i/3zr9Qk.png
Incorrect lighting in forge: https://kek.gg/i/x7c76.png
Optifine or other mods are not involved.
fml-client-latest.log: https://pastebin.com/nQCGA9Br