Repro:
1. Pick Rubick, level up Spell Steal and get Aghanim's Scepter
2. Steal Nether Swap from an enemy Vengeful Spirit
3. Die while having Nether Swap
3.1. You now have a Vengeance illusion
4. With the Vengeance illusion, steal Supernova from an enemy Phoenix
5. Wait until your main hero is about to respawn again
6. Before he respawns, cast Supernova, timed so that your hero respawns during it

Result: When your hero respawns, the Vengeance illusion dies as it should. However, the Supernova it cast is not cancel and continues normally, dealing damage and then stunning at the end.

Expected: When the caster of Supernova dies during it, the spell should cancel. This is what happens when Phoenix dies during it, so the same should happen when the caster is an illusion.