Cognitive brownout is the stage before burnout that nobody calls in. A blackout is total; the lights go out and everyone notices. A brownout is partial: the voltage sags, the lights dim, things keep running a little worse, and no alarm trips.
Judgment does the same thing under sustained load. You are still showing up, still answering, still fine, but the quality quietly drops and the decisions get a little lazier. Because you never went fully down, nobody, including you, flags it.
Burnout gets the attention because it is a visible failure. The brownout is where the damage actually accrues. Watch the dimming, not the outage.