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.
Here is a single chest under a cobblestone stair.
It opens quite easily.
My standard procedure for making a enchanting "Lair" when my base is a village with two libraries is to place an iron door on one of them to keep villagers out. Then I "harvest" all the bookshelves from both libraries with silk tough axe to make the enchanting nook in my Lair.
To get more storage room I will remove unneeded material in the ceiling of the library under the roof stair blocks and place chests between top of the walls and the roof stair blocks. The image below shows a single chest, trapped chest, and a double chest. All open easily.
Also I've found that I can stack double chests in a three block wide column as long as one side of the chest has an open block over it.
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There are no dangerous weapons. There are only dangerous people. R.A. Heinlein
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