I had the same issue with an all rosewood neck JP13. The issue was more wide spread than just an isolated fret. I had numerous frets (especially on the treble side of the fretboard) where the "filler" had fallen out. I sent the guitar back to costumer service, primarily to touch up some sloppy fret work, and had them fix this issue also. When I received the guitar everything was perfect. However, not even a month had past when I noticed that some of the filler had disappeared leaving gaps again. I can only speculate but I believe the cause of this is primarily due to pre cutting the frets the same for multiple instruments. The fret tangs are probably cut a tad short as to not have problems from fret sprout on a finished neck like a JPXI. The same pre cut frets are probably the same ones used on non finished necks like your JP15 and my rosewood neck JP13. The fret tangs are just a touch to short and do not come all the way out to the end of the fretboard hence the reason for the "filler". Do to the ever expanding/contracting of the neck I believe the "filler" is literally pushed out of the slot leaving the small gap. I do keep a Planet Waves Two-way Humidification System in my case to help keep the expanding/contracting to a minimum. Can't say if it helps with the issue but it doesn't negatively affect anything.
I would be interested to see what customer service says. I'm not really wanting to risk sending my guitar back from central Kentucky for another temporary fix. As I said earlier this is just speculation at best but I think the only way to permanently resolve this issue is with a new fret job. Having said all that, my JP13 plays absolutely wonderfully. No guitar is ever perfect and if this is the only issue I have with the guitar than I'm a satisfied customer. Hope this helps...
