I haven't played Malzahar much on live, so this might be a normal interaction, and not something to do with his update. I tested this in a bot game just to be sure, spawning voidlings at different times and different distances from turrets, and it is very consistent. If there is a voidling within the turret's range, as soon as the turret's current target dies, it immediately targets the nearest voidling, and will continue targeting voidlings until there are none left within it's range.
This is consistent without regard to the voidling's position - it can be on the very edge of the turret's range while three melee minions are at the turret's base and the voidling will still be prioritized.
Is this intended behavior, or is it some sort of a bug?