Immediately after the update that implements the tech tree into the Keep, I tried to see what the keep is capable of.
After "Tools production II" the time of the blacksmith is "-1.1 min" and doesn't produce any tools whatsoever. Other buildings like kiln or sawmill show positive values (1.4 and 1 respectively).
This is obviously a bug.
I don't know what other info to give beside the usecase to reproduce.
PS: does the distance between the 2 building counts? If so, the blacksmith is right next to the Keep entrance, 1 space away.
Also... most of my people died of starvation after lvl 2 or 3 for techs.
A tooltip on how exactly the extra productivity (they seem to work themselves to death, not eating especially) is obtained might help avoid disasters like mine.
Good luck on bug hunting.