It has happened to me, many times already, that when I exert Glorybringer it doesn't fire the 4 points of damage.
I'm on a windows PC.
The matches in which the exert failed to fire were always ranked (best of three).
There was always a target creature to target and no rules-changing permanent affecting abilities whatsoever.
Once it was targeting Nicol Bolas, The Ravager; another time (different match) it was targeting Terror of the Peaks and a third time targeting some other creature I don't recall.
It's been happening since the last days. And since the last days I'm playing best of three ranked matches, maybe it has something to do with it, I don't know.