Im pretty sure lux and ezreals ults are special channels that cannot be interrupted:
"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.
There is a small area in front of Lux's character model which will still deal damage after being displaced. The rest of the laser however will damage from the original point."
From the wiki, so its probably working as intended.