For a weapon to disappear from the hands of a killed player, it is enough for him to lie down.
I launched a completely vanilla test server, with one mod (Vanilla PlusPlus Admin Tools) for admins and checked in what positions weapons disappear. In all cases, except for the prone position, the weapons remained. You can see this in the screenshots (Lar and M4 are not displayed via TAB)
If the corpse is cut, the weapon will visually disappear