Current behavior: This happened in the second cloakwood area, north of the tree house. I encountered two vampiric wolves (apparently spawned randomly which I've never seen happen in vanilla BG and I'm not sure whether that's a bug as well). Jaheira was using this magical +2 club and couldn't hit them (weapon ineffective).
Expected behavior: Mighty Oak +2 (BLUN37) should work against enemies immune to non-magical weapons. Looking in Near Infinity, club +1 (BLUN38) should also be flagged as magical and isn't.