I noticed yesterday that some of my npcs had lost their bows (and another player mentioned they had the same issue with shields). I tried to re-equip them, but they kept disappearing when I logged out and back in.
I had used common, epic and rare items, as well as epsilon items, and they all kept disappearing.
Using the NPC forge interface didn't help, and neither did using the command .ph forge npc weapon ###### 2
The bow re-appeared if I used emotes or spells related to shooting with a bow, so it clearly was hidden somewhere - they were only invisible while unsheathed/sheathed (not to be mistaken with the invisible-sheath type of weapons, I didn't accidentally use those).
I was able to bubblegum-fix it by using custom epsilon bows that sheathe like shield, and put them in the main hand slot instead of ranged slot. Using anything else but sheathes-like-shield would just make them sheath wrong, and putting it in the ranged slot would turn them invisible.