Date & Time: 09/29/20
Frequency: Occurs consistently and has been since at least the Stormblood patches before Shadowbringers.
World Name: Balmung
Character Name: Berteaux Braumegain
Class/Level: Red Mage/80
Party or Solo: Either

Steps:
1. Cast a spell with Red Mage, causing Dualcast to activate.
2. Activate Sprint.
3. The Dualcast proc disappears.

I am assuming that this isn't intentional, and it makes maintaining offense while moving a bit frustrating.