I was able to reproduce this same issue with my spellcasting system. My fireball would just go through the landscape and never fire a hit or overlap event. It was very odd. I fixed it by dragging my Sphere Collision component to be the DefaultSceneRoot. As soon as I did that, the hit events started firing. Overlap events still do not fire, but hit events are good enough for me as I can use that event to trigger my explosion.
If you want to reproduce my results, create a ThirdPerson blueprint project. Add a landscape. Create a new blueprint derived from Actor. Add a sphere collision component. Change the Collision Preset on the Sphere Component to “Block All” Add a Projectile Movement Component. Set the Projectile Initial Speed to 2,000. Create an on hit event in this new blueprint that prints out the name of the object it collides with (Other Actor → Get Object Name). Now fire the projectile and you will notice it goes through the landscape. Open the projectile blueprint back up and drag the Sphere Collision Component onto the DefaultSceneRoot (this makes it the scene root). Now fire the projectile again and it collides with the landscape and prints “Landscape_0” on the screen.