I tried to reproduce and debug this, but the bug is somewhat slippery.
It does not happen in 1oo% cases, I succeeded to reproduce it only once max twice per day.

I still do not know the exact cause of this, but when the looted weapon (fnx45 in my case) does not fire, it does so because it has no muzzle set. _currentMuzzle is set to -1, but the number of muzzles is 1.
So there are muzzles, but none of them is set as current (by SetCurrentMuzzle or similar mechanism).

I do not see yet how this applies to fire axe for example, as it has no muzzles.