This happens frequently enough to warrant bug mention.
Assault rifle grenades can be sometimes involved to launch during heavy combat - there are energy, distance/range, hostile targeting (reticle, LOS and target death) factors that may cause a grenade to blow up in your face.
Nonetheless, grenades, at least Essence Grenades, are frequently protracted in launch - seconds can pass by waiting for the launch-ability animation to start/finish. Ground effects can even occur before the animation has processed.
Despite the tool-tip describing an instant ability.
At present I can think of no other weapon ‘instant’ ability to be so delayed.
Additional problem is when, even despite the ability activating, and waiting through the protracted animation, the grenade launch still fails and blows up in your face - for none of the player errors given above.
Example:
You start using Essence Grenades.
(Critical) Your Grenade Detonation hits (Normal) Mercenary Veteran for 812 physical damage.
Your Grenade Detonation hits you for 1173.
Flip-side may be the opportunity to have the launch activate twice off of the one grenade.