Not a bug, that's by design.
These are a recreation of the old, old, old farms from forestry in mincraft v1.4.7 and earlier. They used to build farms like that. The space in the middle was needed for additional machines and piping anyway, back then... (read more below)
Here look at this image:
link
Then came the multiblock farms. And now in 1.12, unexpectedly, the old farms are
somewhat back again. (but not exactly the same)
The linked screenshot was taken years ago. In that version of forestry, you needed a separate block for harvesting and feed the farm with stuff that was produced by other farms.
To make all the farms work endlessly without manual input, you needed to make a complicated setup where all the different materials are produced in balanced amounts. This required the use of multiple pipes, engines, sorting mechanism, buffers.
The newer multiblock farms where way easier to deal with, and the current incarnation of the "old style" farms are also easier to deal with.
In modern mods, the sorting filters are usually a part of the duct/conduit itself, rather than having their own block, and buffers aren't needed because most transport systems will send exactly what is requested and stop sending when the target inventory is full. Back then most people used buildcraft pipes. Redpower tubes worked more like modern TD ducts do, but they still required filter blocks. And the more advanced versions of those filters required power. A different power than forestry farms, so that means even more wiring.
So the creator of forestry designed the farms as a ring around the machinery, leaving space to build all that stuff.
The new "old style" farms mimic the style of the old farms, but it is not the same.