It has happened 3 times since last update. Cyprian’s buff is still on, but the enemies attacking him didn’t encounter the reflected damage.
They tried to fix this bug
but they made an error. There’s no word yet that they realized they’ve made an error.
So we have this suggestion now:
The devs have said this issue of reflected damage from a fatal blow to a Riposte hero will be remedied in version 15.1 due soon
The “fix” applied to stop the dying hero countering has, apparently, adversely affected a lot (most/all?) riposte specials in the mean time
Thank you for the good news!
Fix coming with version 15.1