Tracker's Knife with sightstone (bug?)

Not a bug. The number of wards you can have out is limited, no matter where they come from. So you could have Tracker's Knife, Sightstone, and the warding totem Trinket, but still only able to have 3 wards out at a time.

Thought the same thing at first -- took some messing around in a custom game to figure it out. They both have the same "unique passive" and unique passives don't stack. That's why they synchronize and both show the number of wards you'd get from sightstone alone.