Li ming: right click cancels channel disintegrate
This isn’t a bug. This is intentional. If this was a bug, imagine how clunky it would feel to have only one way of canceling your ability!
Stationary channeled abillities include:
Li-Ming’s Disintegrate (your case), Gul’dan’s Drain Life (W), Whitemane’s Inquisition (W), Chen’s Fortifying Brew (D), and more.
It’s functioning as intended. Li Ming is already an incredibily powerful mage - being able to move around while channeling her death beam would also be way over powered (especially since she can take a level 20 perk to add slow to the beam!).