It doesn't do it on the gold myr squad(the one you can lust toxin or tranq) though, and doesn't do it on the zil village either or the zaika squad in Dhaal's dungeon, all masses of entities. Which leads me to believe that this is in fact a bug, because why here but not everywhere else? Also doesn't do it on the turrets immediately after.
Cleave works differently depending on the fight:
Vs 'Group'* fights, it makes one attack against each enemy that isn't your primary target.
Vs 'Plural'** it gives 1 extra attack, because it has no way to know how many extra targets there are.
* Fights with multiple individual combatants. EX: Rats Raiders, Zaika Gangers
** Fights that represent a group as one enemy. EX: Auto-Turrets, Naleen Mating Ball, Raskvel Work Gang, Pratorians.
Going back to an old Flash save, it works vs the Zil Village fight.
(Level 7):
You land a hit on the zil tribe with your lightning rod! The zil tribe’s shields crackle but hold. (S: −9)
You land a hit on the zil tribe with your lightning rod! The zil tribe’s shields crackle but hold. (S: −9)
Level up to 8
You land a hit on the zil tribe with your lightning rod! The zil tribe’s shields crackle but hold. (S: −21)
You land a hit on the zil tribe with your lightning rod! The zil tribe’s shields crackle but hold. (S: −20)
You land a hit on the zil tribe with your lightning rod! The zil tribe’s shields crackle but hold. (S: −21)
Naleen Mating Ball
You land a hit on the dozens of naleen with your lightning rod! The attack directly impacts the dozens of naleen! The dozens of naleen is stunned! (H: −26)
You land a hit on the dozens of naleen with your lightning rod! The attack directly impacts the dozens of naleen! (H: −26)
You land a hit on the dozens of naleen with your lightning rod! The attack directly impacts the dozens of naleen! (H: −26)