Temporal Rift - Uncastable when on low spirit

I don't know what causes this but sometimes when you support Temporal Rift with a spirit gem like Clarity, if your spirit is low (but still positive) it just will stop letting you cast Temporal Rift.

As a user it prevents me from using all of my Spirit if I want to continue to use this skill.

Example/Steps to reproduce: 19 Spirit left unreserved. Socket Clarity into Temporal Rift. 9 Spirit Remaining. Temporal Rift is now uncastable. If I put on a necklace that gives me 15 more Spirit. I can now Temporal Rift again. Both scenarios I am still positive on Spirit and have not used all of it. Only one scenario lets me cast it.