I am not 100% sure but I think it is most likely caused by the way you added the offset.
You get your actor location and add 100 to X and Z. Now Z is totally fine and doesn’t have any influence but X especially when rotating depending on the actor rotation means that your projectile should spawn 100 units away from your character with the offset to wherever the character is facing which looks exactly like what I could see in the video.
It was not actually the movement but the rotation of your character which defined the offset.
Try not doing the “Rotate Vector Around Axis” thing but rather a “Get Direction Vector”, give it a static length and your projectile will always spawn in relation to your cursor instead of your character rotation.