One hell of a bump, but it did bring this awesome mod to my attention, so thank you for that! When I have had issues like this I've wanted to eliminate, I've used paint to paint the insides of the cap.
This is much easier with no dry time, and it's not permanent! Definitely noted down in my tips and tricks book. I'll have to keep this in mind. The fact that it improves the underglow is really cool as well!