So, I was playing some operations on Substantial difficulty, and was in the mission "Inferno" when this bug occurred. Placed down a melta bomb, then detonated it in a crowd of tyrannids. Went down shortly afterwards. When I was revived, the prompt for detonating the melta bomb stayed on my screen, and completing the prompt did nothing other than make my character play the animation for detonating the bomb, without any melta bombs going off. Did not live long enough to see if the bug affected other things, as upon dying and respawning, the persistent melta bomb detonation prompt disappeared.

Edit: Upon reviewing the footage the Steam captured of this incident in preparation for uploading it to you guys, it looks like getting downed was not part of the bug at all. It just looks like the prompt got stuck on my screen after I activated the melta bomb.