Repro:
1. Pick Earth Spirit and level up Boulder Smash
2. Create an enemy Rubick and Spell Steal Boulder Smash
3. Place a Stone Remnant
4. Cast Boulder Smash on the Remnant with Earth Spirit
5. While the Remnant is being knocked back, cast Boulder Smash on the remnant with Rubick

Result: Rubick's cast does apply the damage and stun regularly in a line the remnant would have travel. But the Remnant itself does not get smashed, it continues traveling where the first smash sent it. Also, the first smash's aoe damage and stun are not canceled.

Expected: Rubick's cast on the remnant (or unit) should cancel Earth Spirit's cast completely (aoe damage, aoe stun and the remnant's movement) and the Remnant should start moving in the direction Rubick smashed it, damaging and stunning Rubick's enemies. Imagine it like passing a ball back and forth between 2 player.