I have a 0% experience bonus on my level 70 Demon Hunter and my level 56 Crusader.
And yes, I've verified that this is not just a display issue; my heroes gain the same experience killing monsters that they did yesterday.
I would like to know whether this 0% bonus is intended before reporting a bug to reclaim my bonus tomorrow or whatever Blizzard team decide to do.
By the way, if there is a reason explaining why a player could have 0% bonus I would like to know the url of the official page where it is explained.
Best Answer
There is a reddit post that seems to translate the announcement from Blizzard's Korean Site, but it can also be found on the English site here:
(emphasis mine)
It does not say anything about an Experience Boost. I think it is included (as 0%) in the tooltip is that this buff (icon) and tooltip use a standard template.
It definitely is not a bug, as you are not supposed to have an experience boost, just Legendary Drops and Rift Fragments.