The fearful mind cannot comprehend eternity, and eternity is an attribute of Soul. Human fear, generated by this mind, is time-bound and is anxious about the future. Since twin flames have all of eternity to work out their interplay, anxiety about the future does not apply.
Fear does lower our vibrations to where we are not emanating a pure, clean field. But it has no power over the everlasting-ness of the twin flame, soul bonding. That can never be touched by the flickering restlessness of the human mind and its transient fears.