All of the original Iron Front maps + their upgraded versions and Invasion 1944 maps (+ their upgraded versions) seem to be using Arma 2/Iron Front era cloud (or weather??) system (or then the reason for the issue described herein is something else). Because of that "system" (or for some other reason) it is not possible to get a sky that is completely covered with clouds.
In Arma 3 cloud coverage changes according to the current overcast (setting). Changes to the overcast setting and its effect on the cloud coverage can be made and observed in the Arma 3 editor (see Attributes/Environment). Overcast (setting) can be adjusted between 0 and 100%.
When testing with Arma 3 maps (+ some of the official Iron Front maps that don't have this issue) clouds start to form when overcast has been set to around 9%. Before that 9% setting the sky is clear (cloudless). The clouds begin to cover the sky the bigger the set overcast value is and at over 85% there is no visual opening among the clouds. The GIF below shows how different overcast values affect cloud coverage in the map Aachen outskirts
In the original Iron Front maps there's always some clouds even if the overcast is set to 0%. Between 0 and around 38% the cloud cover stays pretty much the same. Between around 40% and 60% some changes in the cloud cover can be observed. From 60% towards 100% the clouds only change from light to dark but cloud cover does not change in any way. Even at 100% overcast setting the sky won't be completely covered in clouds.
The GIF below shows how different overcast values affect cloud coverage in the map Staszow
The maps listed below are not affected by the issue described herein (list updated on the 17th of September, 2020):
[Winter] Baranow (BaranowWinter)
[Winter] Battle of the Bulge (PLR_Bulge)
[Winter] Colleville, France (CollevilleWinter)
[Winter] Ivachev, Ukraine (IvachevWinter)
[Winter] Panovo, Ukraine 1944 (PanovoWinter)
[Winter] Staszow, Poland 1944 (StaszowWinter)
[Winter] Merderet river (I44_Merderet_Winter)
[Winter] Neaville (MCN_Neaville_Winter)
1944 Operation Crossing Point (I44_merderet_koth)
Aachen outskirts, Germany (SWU_Aachen_Outskirts)
Baranow, Poland (Baranow)
Bray-Dunes, France (SWU_Dunkirk_Bray_Dunes_1940)
Colleville, France (Colleville)
El Alamein, Egypt (SWU_Egypt_El_Alamein)
Ivachev, Ukraine (Ivachev)
Merderet river (I44_Merderet)
Merderet river v2 (I44_Merderet_V2)
Merderet river, France (I44_Merderet_V3)
Montherme, Ardennes 1940 (SWU_Ardennes_1940)
Montherme, Ardennes 1944 (SWU_Ardennes_1944_Winter)
Panovo, Ukraine 1944 (Panovo)
Pella region, Greece (SWU_Greece_Pella_Region)
Sark, Channel Islands (Hyde_Sark)
Staszow, Poland 1944 (Staszow)
Streets of Berlin, Germany (SWU_Germany_Streets_of_Berlin)
Villers Bocage, France (GuntherS_Villers_Bocage)
Tobruk, Libya (iron_excelsior_Tobruk)
Volkhov forest, Russia (SWU_Russia_Volkhov_Forest)
Can the maps mentioned in the first list be upgraded to the Arma 3 weather system so that changing overcast setting would affect cloud cover the same way it does in official Arma 3 maps (+ in some of the official Iron Front maps that don't have this issue)?
If the maps mentioned in the first list are already using Arma 3 weather system then what might be the reason that changing overcast setting does not affect cloud cover the same way it does in official Arma 3 maps (+ in some of the official Iron Front maps that don't have this issue)?