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Powder "burning" may (or may not) essentially cease as the bullet travels down the bore due to drop in pressure, BEFORE all of the powder is burned. That happens because the rate at which the flame front advances through the powder kernals is a function of the pressure in the gases surrounding the kernals, and that pressure is dropping as the bullet moves to make more space for the gases and the powder does not burn fast enough to keep pressure up (after the peak).
In revolvers with slow-burning powders like 2400 and 296, it is not uncommon to find amber-colored kernals of unburned powder left in cases, barrels and on the table/floor in front of the shooting position, usually when using those powders for less than full-power loads. In fact, W-296/H-110 is notorious for "squibbing-out" and leaving the bullet in the bore, as well, when the pressure from too-light charges never gets high enough to keep the powder burning.
So, it is possible to shoot unburned powder out of the muzzle, which does NOT light when it reaches the air and more oxygen.
On the other hand, some faster powders like Power Pistol are notorious for muzzle flash, even though QuickLOAD calculates that they are 99+% burned in a revolver with a 6" barrel.
So, it is possible to get a lot of muzzle flash when only the resulting powder gases reach the muzzle.
And, when the barrel is short enough and the powder is slow enough, it is possible to shoot burning granuals of powder out of the muzzle along with the gases.
"Flash-suppressed powders" are formulated (with additives) to make the gases that reach the muzzle essentially non-flammable in the air where they are exposed to oxygen. So, I am assuming that MOST of the muzzle flash is normally the combustion gases of the powder burning when mixed with the oxygen in the air outside the muzzle. But, there are certainly other potential contributors.
SL1
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