Ahri's charm does not cancel lux's ult

Lux's ult isn't a channel it's just a long cast time. So it won't be interrupted like that

This is intended behaviour, not a bug.

From the Wiki for Lux's ultimate:

Cast times cannot be interrupted by crowd control effects. During the cast time, Lux will ignore crowd control effects but still suffers from them if the duration persists.

If Lux is displaced by a knockback or pull effect during the cast time, the laser will fire from the original point of casting.