I searched the forum before posting this and I found another topic with similar problem but not much detail and there was no solution or reply from the developers.
This is the one:
My observations lead me to believe that using shield throw with the “Aegis”, “Manifest Inspiration” and “Focused Campaign” specializations on a despawning manifested weapon from “Forge Strike” sometimes causes the shield to despawn/disappear as well, resulting in it never coming back in order to refresh the skill’s cooldown, like “Focused Campaign” specialization says. This is not game breaking and doesn’t happen “too” often but it still is not working as intended in my opinion.