This is an easy-to-reproduce bug, so hopefully it wont be too much trouble to track down.
When the Combustive shot projectile is in the air, the max range (1200) is calculated from your location at the time of IMPACT, not from where you fired it. In other words, if you fire at a location 1200 away, and then while the projectile is in flight, you move further away and increase your range to 1500, your projectile will fall 300 short of where it was meant to hit.
How to reproduce easily
1. In Lions Arch, go to the target dummies near the WvW portals. Build up adrenaline.
2. Press F1 to get the green targeting circle (may need to adjust ground targeting options) and pick a fixed location at max range. _I find it easiest to center the shot on one of the dummies or a nearby NPC
3. Fire the combustive shot, and while the shot is in mid-air, dodge-roll backwards twice.
In all my tests, the projectile visibly continues to where it should go, but the burning/fire field ends up falling short, presumably because it calculates max range at the time of impact.
