The World of Avlis

I have a weapon that does a bonus to slashing damage, it does d4 slashing.

when casting magic weapon (or other greater ones, teste donly to +2) it only adds to the attack bonus

shouldnt the enchantment stay seperate from this other d4 perma enchant and thus give:

weapon with magic weapon cast on it
+1 ab
+1 slashing
+1d4 slashing

but in the character sheet it shows

+1 ab
+1d4 slashing

Is this they way it should be, bug, another lame thing NWN has?

the way it should be.

similarly, if you have a scimitar (slashing) that is +5 and has +d4 slashing dmg, it will only do the +5, not both.

However, if you have your scimitar (slashing) +5 and it does +d4 bludgeoning, it will do the +5 and the +d4.

Like damage types do not stack. Cast flame weapon or darkfire on a hot/burning weapon some time to see another example of this.