this lora may generate images with messed up weapons as you may have already imagined, but there is still a low chance of generating perfect images. I'd suggest pushing the weight higher for better consistency. Placing katana as your positive prompt may interfere if your characters have their own weapon which is not a katana.For cases like these, the best solution so far is to remove the prompt "katana".

Play around and find the best weight for yourself, as always have fun and go break some eggs!