This is a known issue with hit boxes.
You should have totally gotten a headshot but right now there’s an issue with brimstones head hitbox when he’s in the menu for smokes which he is in your clip when you shot him. There’s some other animations like defusing and whatnot affected.
I saw some one from riot address this in another thread.
I’m sorry you got screwed out of a sweet dome shot tho.
Thanks for the video /u/Dalorick and the comment here /u/howtotailslide - we are looking into this issue!