Mana Tempest Bug

Mana Tempest is not working correctly or at least not as per the tooltip.

Current if you stand in Mana Tempest and cast a single ability it adds approx 30% of the abilities mana cost to the Mana Tempest cost per second.

If you cast any additional spells it, waits around 1-2 seconds and proceeds to increase the mana cost rapidly, looks like it's adding 30% of the mana cost EACH second to the cost per second of Mana Tempest.

Are you going to fix this anytime soon? This should be a simple hotfix issue.