Whether or not muzzle flash is visible to the human eye depends on a number of factors, such as the powder used (slow-burning or fast), the charge of powder, the brightness of ambient light. Even when all these factors are nominally the same, a given load may show muzzle flash, or not, seemingly at random.
Of course, we're talking about night shooting, where any flash at all tends to be readily visible. Preventing flash entirely at night (with live rounds) is a matter of adjusting the load such that the entire powder charge combusts before the bullet exits the barrel. This is especially difficult with pistol loads due to the short barrel lengths.
Adding a film camera to the mix makes visible flash less likely. In simple terms, muzzle flash tends to be shorter than the period between subsequent frames. If so, whether muzzle flash will be visible or not depends, in part, on whether the weapon's discharge happened concurrently with a frame of film being exposed (or video captured).
If there is nothing syncing the weapon's trigger to the camera's shutter (why would there be?) - or alternatively a strobe in a controlled (dark) environment, then it is to be expected that a proportion of fired rounds would occur during exposure, and the rest between exposures.
This is why sometimes you'll see footage of machine guns where muzzle flash appears only intermittently.
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