The grounded condition from Lockdown doesn't seem to go away from friendly champions even if they move out of the zone of the skill and even if the champion with the ability dies.
Question: Was the champion perm effected by Lockdown on normally unaccessible ground and thus unable to move at all or was it on regular ground?
Alright then you may be right. I have only seen it on impassable terrain though it was likely coincidence.
It happens regularly and consistently, I already reported this and the way to reproduce in the compilation thread and was acknoweledged already.