Yes it's true that modern artillery doesn't produce as large\huge\visible muzzle flash than some old artillery piece, but could someone explain me this:
Just for the sake of example, let's imagine a groundscape like this and you're at the ground level.
Under daylight how visible you think even notable muzzle flash would be considering the artillery battery could be 5km+ away from the frontline due trajectory (and safety) reasons? 10km? There's plenty of diffrent obstacles blocking your line of sight and even if you're on high ground\some tall building dust, rubble, poor weather conditions and how human eye behave with diffrent lightning conditions would make it so that even if you see it, it's not gonna be that visible. Night conditions are obviously diffrent, as it's easier to spot any source of light(ning) in dark area than it's in middle of a daylight.
Now while IRL you could - under certain conditions - see something going on during daytime, considering scale object syndrome that objects appear smaller on screen than they "would" on human eye, it would make the problem even more notable.