In 3rd person view when standing on firm ground, looking up and throwing a grenade it hits the ground behind your character. Could be somehow caused by the camera clipping thru the ground. The grenade thrown animation continues but there's no explosion at the end of it.
For me, this happens when I am with in 5723059687250967 feet of any solid object; wall, arch, tree, even rubble. The grenade detonates behind me or at my feet. As someone who was an Expert Grenadier in the US Army, I know for certain that this is not how grenades work. I've had people say, "Just learn the mechanics." but that is a cop out.
This is probably caused by the same thing as when your companion stands too close behind you while you're shooting and your shots hit them, makes no sense really.