This is clearly not a bug, but by design, for balance reasons.
There's no reason stuff unique to a civ has to be strictly better than their non-unique counterpart. The Arabian camel archer, for instance, is better than the knight in some ways (can bombard) and worse in others (weaker when attacked and cannot take cities) because it is ranged.
The longhouse is already usually better than the workshop; +1 hammer per forest is better than a flat 20% for buildings only, especially considering where the Iroquois tend to spawn (in wooded areas). Having the 20% on top of the +1 hammer is unbalanced.
Also, this is not new to 1.0.1.141; the longhouse has never, to the best of my knowledge, given 20% building production, not in the current version or any earlier versions.