Boucing projectile spells are usually able to bounce to spell immune units, and also bounce off of them. This is to prevent them from completely fizzleing only because one target turned spell immune before the projectile hit.

As a comparison, other bouncing projectiles can bounce off of and bounce to spell immune units: Paralyzing Cask, Mystic Snake and Chain Frost.

Repro1:
1. Pick Bounty Hunter and level up Shuriken Toss and Track
2. Have 2 enemy heroes, one of them with BKB
3. Cast Track on the non-BKB enemy
4. Cas Shuriken on the BKB enemy
5. Activate the BKB before projectile impact

Result: The projectile does not bounce off of the now spell immune enemy. It completely disappears.

Expected: The projectile should bounce off of them, without harming it in any way (no damage, no stun). Spell immunity should not disrupt the shuriken's bouncing.

Repro2:
1. Pick Bounty Hunter and level up Shuriken Toss and Track
2. Have 2 enemy heroes, one of them with BKB
3. Activate BKB on the enemy
4. Cast Track on the BKB'd enemy
5. Cast Shuriken Toss on the non-BKB enemy

Result: The Shuriken does not bounce onto the spell immune tracked enemy. It completely ignores it.

Expected: The projectile should bounce towards the tracked enemy, regardless of spell immunity. It should not harm it though (no damage, no stun). From here, it should be able to bounce off to other tracked heroes again.