Warpath is supposed to grant all illusions under the player's control a stack whenever casting a spell. However, it only grants Bristleback illusions a stack. This means if you have illusions of other heroes under your control, those illus do not gain a stack.

This is mainly relevant for Morphling (a Hybrid BB casting spells doesn't grant stacks to your replicate or manta illus), and for Lotus Orb (reflecing Disruption, the illus dont gain stacks). It also matters in Ability Draft.