I just had this happen with oak wood when cutting down some trees.
The item id of normal oak logs is 17, and the item id for this "mystery" oak log is 17:8
Yet if I place that block, then break it, it becomes a standard wood like like the others.
They all came from thew same batch of oak trees in my farm.
I have also had this happen with the leaf blocks that drop due to the farmer's passive skill when normally breaking leaves. I'd end with a stack of oak leaves and a single oak leaf block that wouldn't stack with the others.
EDIT: Here it is happening with oak leaf blocks. The stack of 4 is item id 18:8, the single block is item id 18.
Perhaps it has something to do with the various class skills that give double drops for breaking certain blocks?