Dank Null Version:
p455w0rds Lib Version:
Forge Version:1.12.2
i think
Steps To Reproduce:
1.create dank
2.put block in dank
3.fail to place said block with dank

Was this tested on Single Player or Server (or Both)? single

Was this tested with just Dank Null installed, if not what is the minimal set of mods to cause the issue? do not understand question, running in roguelike adventures and dungeons...

The question at the end is because some times mods will conflict or interact with each other and cause bugs due to this interaction.
It is to try to narrow down the cause of the bug.
If the bug occurs with only /dank/null installed, then it is a problem with it. However if it only occurs with other mods it could be a problem with any of those mods.
So rather than reporting the bug occurs in a particular mod-pack, it is best to first check if it /dank/null is enough to cause the bug.

However, it appears that what you are describing isn't a bug.
If I recall correctly the default extraction limit is 1, meaning if you only have 1 block inside the unit, you can't place it. That is how it remembers what is there.
Try putting an entire stack (or more) inside and seeing if you can place it then.