Having gone in blind (twice), at first I had thought there was some kind of glitch in the swoop AoE during Mist Dragon's fog phase, because it was either coming out too late to react to, or entirely too early. Obviously, it turns out you have to find the boss in order to make its AoE marker visible to you, but herein lies the problem: the in-game text poorly conveys what we should be doing.
When the Mist Dragon flies away, we're told "The dragon is poised to swoop! Be on your guard!"
Telling me to be on my guard makes me think of having to watch out for where it will strike, like in many other instances where a boss leaves the arena to swoop through it. In no way does telling me to be on my guard incline me to run around in search of the boss. The text hint should be something like "The dragon is poised to swoop from hiding! Find it to reveal its attack!" and perhaps an extra bit of dialogue saying "You found it!" to actually convey that finding them is the correct thing to do. If not the former, then at least the latter to let the player know there is a correct way to handle that phase.