Originally Posted by Blizzard Entertainment
To clarify: this is just a tooltip error. The current build you have should still have a 10 maximum stack of Maelstrom Weapon. In-game, you are probably seeing two different buffs, one that stacks up to 5, and one that stacks up to 10. The one that stacks up to 5 will be hidden in a future build. The Maelstrom Weapon tooltip is just pointing to the wrong place for maximum stacks.
We hear there are some problems with Chain Harvest and this iteration of Maelstrom Weapon, that will be looked into and fixed as well.