That is tremendously unlikely to be the case. WIth a 2.5% drop rate, the chances of it never dropping in 1000 tries is a bit more than 1 in 100 billion (0.975^1000).It's not a bug, it's just a consequence of the item's low drop chance. Beacuse the npc drops are not guaranteed to be displayed(especially with rare drops) Because the mod's method isn't foolproof; It just spawns 1000 of every single NPC kills them and checks their loot.(this happens while reloading and the bar saying "rebuilding loot cache").