As it says on the tin. This bug has been around for at least several months (probably longer), been reported repeatedly, and applies to the following weapons:
Gunsen
All Nikanas - including Umbra Excalibur's Skiajati (will show through the scabbard)
Venka Prime
Sigma and Octantis (only shows in mission and will temporarily disappear at random)
Glaive Prime
Orvius
There are possibly a few other weapons, but these are ones that I