Losing LOS to an ATG

[RNL Tiger]

I have a MG42 and a sniper that have LOS to a "light gun", which appears to them as a spotted marker around 200m away. The gun becomes visible to them when it shoots and knocks out a scout car, they fire at it but it soon becomes a spotted marker again.

1) So the gun is not firing anymore and goes "dark" but why can't my units continue to shoot and kill it since they have now seen it properly. Why do they lose LOS?
2) What units will maintain LOS until the gun is dead?* Or do I need to have it shooting at something for that to happen?

* In my AAR I had noted how a sniper had lost LOS to a 20mm Flak, presumably for the same reason as here, so this time I deliberately got an ordinary infantry unit [MG42] closer to also get LOS and to keep it visible but the same thing happened