You're not the only one. I'm experiencing this bug almost every mission since 34.1.
Reproduction:
* While holding E for auto-melee, perform a mercy kill.
* The mercy kill animation plays and resolves correctly, but the equipped weapon changes to "Parazon" - failing to revert back after the mercy kill animation. The player cannot fire because no real weapon is equipped.
* Changing weapon returns to primary / secondary as normal, re-enabling attacks.
(But, obviously, that's a painful and sometimes fatal delay to reach for the button - particularly on steel path, where you're likely to hit this bug with every eximus.)
Bug happens in any squad (host or client), with any weapon or warframe.