Whispers of Infinity deletes skill costs sometimes?

So, I've been trying the Forbidden Rite of Soul Sacrifice and I noticed that its cost was not being displayed.

As a matter of fact, it had none when being cast.

Upon testing, it appears as though equipping the amulet after removing it will restore the skill cost, but when removing and reequipping the skill gem Forbidden Rite of Soul, the skill's cost would be deleted again.

This appears to be the case for any skills with an innate life cost, but not when the cost is converted from mana to life, such as using the lifetap support.