"As the video shows, certain casts of Spear of Slyvia with Core: Spear of Slyvia do not grant forward guard as the core skill entails.

Piercing Light, Light's Path, Flow: Echoes of the Call, and Double Tap all cause Spear of Slyvia to fast cast without the forward until after the arrow comes out which is strange considering Marked Bloom, Grand Bloom, and Mountain Breeze all fast cast and work the way you would expect them to."

https://www.youtube.com/watch?v=RlZEyC-puRQ