It feels unfair because you weren’t expecting it. Somewhat arbitrary.
I dunno.
(this is still about the example where the player was asked to describe how their attack killed an NPC and they came back with 'shredded'... right? Or am I responding to the wrong thing? My response below assumes it's about that.)
Tacking on an added penalty because of the narration is "almost" the same as something directly mentioned as a 'do not do this' in Daggerheart.
In Daggerheart they specifically call out NOT using a roll with fear to negate the outcome.
So if you roll 'success with fear' - the GM needs to remember that is a success. And not change the outcome.
If you roll 'fail with fear' - the GM needs to remember that is a fail, not a critical fail. And not change the outcome.
You apply that fear downstream or as a followup.
It's less obvious but the same applies to something like a fail with hope - don't turn it into a success. Let the player use the hope downstream.
Mist has the same advice with regards to applying a 'consequence'. Especially on a success with consequence. It CANNOT undo the 'power tags' a player obtains through the success. It can be applied to set the stage for the followup, but it cannot undo the success.
So here you have a player saying
"this is how my success worked - it did this to the enemy."
And the GM is now saying "as a result, you lose access to the result of your success."
On the one hand the GM did it right: they applied the penalty to the followup, not the direct moment.
But it very much feels against the "spirit" of the idea of encouraging narrative play.