Flame Rush has a node in it’s skill tree called “Epilogue” which does so Runic Invocation is casted upon exiting Flame Rush. This mechanic is bugged, since Runic Invocation will only proc maybe 20% of the time if you exit Flame Rush manually, instead of waiting for Flame Rush to end by itself.
From my experience, it will always cast Runic Invocation if you wait for Flame Rush to end/exit by itself, by continually holding down the skill. However, if you exit Flame Rush early, there is only a small chance that Runic Invocation will actually cast. This makes certain builds super annoying to play, since they rely heavily on this mechanic.
For example if you play a build that uses the node “Immutable Order” from the Runic Invocation skill tree, as well as Flame Rush to gain runes while channeling, it is super frustrating when Runic Invocation isn’t being procced when exiting Flame Rush early.