Normally when you apply an enhancement bonus such as blade thirst/greater magic weapon it uses the weapons base damage type to apply the bonus damage.
Steps to reproduce the Bug:
Cast GMW on a bronze Wakizashi or bladethirst on one
Expected Result:
Get +1/2/3/4/5 Piercing damage
Actual Result:
Get +1/2/3/4/5 Slashing damage
Screenshot(s)
Additional Information/Notes
My actual thought on this is that the base damage type is probably meant to be slashing? and this is just a typo, but if its meant to be piercing it is round the wrong way from a bonus damage perspective.