If it was a bug in the game, they would have fixed it long long ago, it has been there since slabs, and I am sure mojang kept it for a reason.
Forge's bugfix is not unlike the recent change that Mojang made that allows you to place torches on the back side of stairs - it makes sense that it is considered to be a solid block from that direction because it looks like one, and a similar fix by Forge means that modders do not need to modify the torch code to make them able to be placed on their custom blocks; likewise, the ability to change block models in vanilla means that you can't just rely on a block's intrinsic properties to determine if it is solid so Mojang really should do the same thing for all sides of a block.
There is a glitch in early versions of Forge that interferes with MoreCommands. Perhaps here too. Try the recommended or later version of Forge.
It is a "bug" in Forge itself:
LexManos commented on May 4, 2016 • edited
The bottom side of NORMAL stairs is solid so it does block the chest. And yes this is a difference between vanilla and forge. And yes we COULD special case vanilla block's and reintroduce this 'feature'. But instead we fix the bug correctly and call it good.
It's much preferred that the game stay as is and hope that mojang will EVENTUALLY fix the bug on their end. This has been the case since ~1.2.5 and only a handful of people care. The rest like the mechanic so we're keeping it.
This is the ONE exception where we change vanilla mechanics and refuse to go back because as I said, it's considered a bug in Vanilla on our end.
Apparently, they see this as the intended behavior and vanilla as bugged, and it does make more sense than vanilla - any block, no matter what it is, with a solid side directly above a chest prevents you from opening it (in a similar manner Mojang themselves recently allowed torches to attach to the sides of non-full cube blocks with a solid face; the face determines the property, not the block itself).