[1.2.6.4.2] Able to use Weapons on Diplomacy Check vs Leng Spiders

I am sometimes, although seemingly not always, able to use a weapon to add dice to the diplomacy check to defeat a Leng Spider.

It seems to be most common when a character plays a weapon then Seelah plays a Blessing of Iomedae. Sometimes the game reverts the combat check to a diplomacy check without removing the weapon.

I have a screenshot, which I will try to add later.

Playing on fully up-to-date iPad Air 2.