The problem is really simple. For starters, I'll describe the step-by-step order of events that -may- happen:
- Fire III, Astral Fire III is applied
- Cast spells till roughly 400 MP
- Cast Flare (with or without Swift Cast)
- Transpose immediately when cast finishes
- User is depleted of MP and is stuck in Astral Fire III
What I suspect that happens is that Transpose takes effect before Flare refreshes the Astral Fire effect, even if Transpose is used -after- the Flare. But if the user is quick enough, Transpose will grant the user Umbral Ice before Flare refreshes the Astral Fire effect (this is also observed in the battle log as well as the buffs being applied on the user)
This doesn't happen often, however. Replicating this "bug" may take a few attempts.