So I logged the state of this variable before, during and after server initialisation.

["b/a", bis_fnc_init, missionnamespace getvariable "bis_fnc_init", uinamespace getvariable "bis_fnc_init", _this, getclientstate]

"b" - before A3\functions_f\initFunctions.sqf
"a" - after A3\functions_f\initFunctions.sqf

result:

["b",any,<null>,<null>,[2],"NONE"]
["a",true,<null>,true,[2],"NONE"]
["b",any,<null>,true,any,"MISSION RECEIVED"]
["a",any,<null>,true,any,"MISSION RECEIVED"]

Not that it is resets somewhere before 1st and 2nd call to A3\functions_f\initFunctions.sqf, it fails to explicitly exist in mission namespace.

I have noticed this thing about mission namespace. Even though in most cases

a = 2;
is the same as
missionnamespace setvariable ["a", 2];

it is not always the case. Something has to be done about this unpredictable scripting behaviour