Excellent tip , I haven't made any of those style pens yet and had a hard time figuring out what you did . I'll be honest , at first I was just skimming over what you wrote ,and just looking at the pictures . Your fix looks very professional , like it was meant to be . I'm guessing there's some type of gap in that space .
Jerry, You are correct, there is a space between the edge of the cap and the inner barrel where the threads actually start. This is the area where you have metal to metal contact when the pen halves are mated. Since the metal to metal contact provides no gripping effect the pens unscrew easily.
I filled that gap with silicone and now you have metal to silicone to metal which gives you a gripping seal.
I took another photo to show the before and after look.