So I killed the Tiefling Karlach.
And before cutting her head the option of studying her tattoo is offerer with an Arcana check.

But if you leave the body with it's head on it. And Click again on it: it will restart the dialogue and offer you to do the Arcan check again even if you failed it before. Hence you can reroll until success.
I guess it's a mistake

i had a similar scenario with the dead boar on the road. i believe some kind of flag needs to be set on world objects/corpse containers