Hey guys. After some testing and reading the function scripts, I figured out the missing attachment issue has something to do with the missionNamespace variable "bis_fnc_arsenal_data", which stores all available virtual items.
Here's my observation:
The function BIS_FNC_Arsenal forces into PRELOAD mode when the said variable is NIL.
After the 1.94 update, when the game is saved, somehow the variable only saves maybe partially. As a result, on loading a saved game, the missionNamespace variable "BIS_FNC_Arsenal_data" is not NIL, thus PRELOAD mode doesn't activate when opening the arsenal. All attachments/compatible mags are missing.
SO, the solution:
Edit your scripts so that every time the variable gets cleared first before arsenal function is called:
missionNamespace setVariable ["bis_fnc_arsenal_data", nil];
["Open", true] call BIS_fnc_arsenal;
It takes longer to load though. Not ideal, but its working for me.