I rather see a problem in the diffuser - it could melt?
cistallus has already stated that he tried using the cap on all the modes and the cap did not melt. I have a Preon Ti I and also use a Chapstick cap as a diffuser for a while now. I've previously used the diffuser whilst on high mode until the battery ran out - it did not melt. Note that my power source was a nimh battery and not a 10440.I just used a Chapstick cap on my LF2X with a fully charged 10440, it works fine on low, medium, high. If you want to run it for a while on high with no heatsinking and risk overheating the LED, the little diffuser cap is not going to make any significant difference in how hot it gets. I let mine get too hot to hold, and the Chapstick cap does not melt. On medium it doesn't get too hot and makes a good area light. (Although the LF2X comes with a better diffuser).