[Fizzlemizz]

BreakupLargeNumbers has been broken for a while even though it is still used in the stock UI code and not yet been replaced by FormatLargeNumber.

If you are putting it into an addon of your own that loads at startup then something like:

hooksecurefunc(StatusTrackingBarManager, "AddBarFromTemplate", function(self, frameType, template)
    if template == "AzeriteBarTemplate" then
        hooksecurefunc(StatusTrackingBarManager.bars[5], "UpdateOverlayFrameText", function(self)
            if ( self.OverlayFrame.Text:IsShown() ) then
                local xp = self.StatusBar:GetAnimatedValue();
                local _, xpForNextPoint = self.StatusBar:GetMinMaxValues();
                if ( xpForNextPoint > 0 ) then
                    self.OverlayFrame.Text:SetFormattedText(ARTIFACT_POWER_BAR, FormatLargeNumber(xp), FormatLargeNumber(xpForNextPoint).." "..math.floor((xp/xpForNextPoint)*100).."%");
                end
            end
        end)
    end
end)