When using Disintegrate and having the Laser Focus node you should no longer require mana to continue channeling, but if you have other abilities that drop your mana into the negative your character will continue to stay immobile in the channeling stance but you are no longer actually channeling Disintegrate.
Either the skill should indicate that Disintegrate requires positive mana to maintain channeling the beam with Laser Focus and cancel the character channeling animation and immobilization penalty after your beam drops, or the beam needs to remain channeling when your character drops below positive mana.