This may be true in the main game, but I didn't notice as none of my characters used a longbow.
The Longbow +1 gives bonuses of +2 THAC0, +1 Damage. The Longbow +2 gives bonuses of +2 THAC0, no damage. Obviously, this makes the +1 strictly inferior to the +2, which I would assume to not be the intended behavior. I can't recall exactly what the correct values are, but someone with better knowledge of the system can say those numbers. Based on the difference between the regular Longbow (+1 THAC0) and the Longbow +1, I would assume the correct bonuses on the Longbow +2 would be +3 THAC0, +2 Damage. This seems an easy enough fix, just correct those 2 numbers.