Jim: Actually it IS a known BG2 bug... simply one that there doesn't appear to be a fix for. See:
http://www.baldurdash.org/buglist.html
Search for "Weapon specialization" to find the pertinent bug entry.
This is a completely different situation from Grand Mastery, for instance, where the BG2 programmers deliberately altered the AD&D 2nd Edition rules for "game balance". (Search for "Grand Mastery" on the same page to see the difference.)