Yeah PLease.

got this problem too

This still exists!???

So I actually found out the issue in regards to the primary weapon disappearing from the void. This bug is viewable even in Zeus.

How the mod works:

From my understanding regarding this mod, the mod creates a container (through the object known as "Ground") in which your primary weapon is stored. This container is attached to you and modified so it gives the appearance. This is also the reason why the primary weapon is an invisible object known as "Rifle" that you see in your inventory, because it needs a placeholder. Once you switch back to your primary or this invisible "Rifle", the mod checks and swaps the invisible "Rifle" out for the primary weapon you had primarily.

The bug itself:

The reason why your weapon effectively disappears into the void and you instead end up holding the invisible "Rifle" is because the container that holds your primary weapon is destroyed. Therefore, when you switch to a secondary or a launcher, the mod is attempting to put the weapon into the container, but the container doesn't exist. Since it doesn't exist, your weapon is basically gone.

Causes for the bug:

The bug happens when the container is deleted. So, it is very likely another mod might be deleting the container or modifying the character, or a mission is doing it. I do have an explanation for this.

I've personally encountered this bug in both Antistasi and Dynamic Recon Ops.

What happens sometimes is that when you are engaged in combat, the container sometimes just "dies". You know how when you open up Zeus, and when a vehicle/person/item dies, it turns black in Zeus?

That happens to the container itself. That isn't much of an issue BY ITSELF, but it does pose an issue when a mission modifies the character.

For example, Antistasi has a garbage cleaning feature, which deletes all destroyed entities to save for performance. Because the container that holds your primary weapon from this mod dies, the garbage cleaning feature of Antistasi detects it and deletes the aforementioned container.

It doesn't happen if you have nothing in your binoculars slot. Otherwise, it keeps trying to switch to the binoculars. In all likelihood, if your weapon disappears completely, you character has dropped it.

Edit: It may also be an issue with shouldering the weapon. Double weapon may also be conflicting, but I'm not sure.

Edit: It also is sometimes connected to whether you have a launcher on your back and/or are prone.