The tooltip for Magma Starter in the Flame Rush, skill tree reads:
[…] and only detonates when you end Flame Rush, alongside any other active Volcanic Orbs
However, this is not happening.
How to reproduce:
Specialize in Flame Rush and spend one point in Magma Starter
Cast Volcanic Orb and then cast Flame Rush immediately after. End the Flame Rush channel before the Volcanic Orb detonates. Watch how the Volcanic Orb you cast before Flame Rush is not detonated by Flame Rush ending.