I don't really know exactly when and how it happens, but sometimes when I switch from throwing spears to melee (Katana), I keep using spears, without depleting my stock. My throwing hand seems to be holding an invisible spear. As soon as I throw it, it becomes visible. The number of spears is not going down though.

Is this a known glitch or could it be due to a mod? I have a mod installed, that lets you pick up spears, that do not hit an enemy. Could that cause the glitch?