I am not sure if it is actually a bug or not because it can be argued both ways but if you hit a One with the Nora unit with an attack from Cleave it does not gain Immunity to the cleaves damage type. On one had nowhere does it state that Cleave makes a basic attack against adjacent enemies it just says attack but it may be seen as implied by some players. Reproduction: My Bastion was adjacent to a Goblin Spellkaiser and a Lonx Hunter. I attacked the Hunter and hit the goblin with cleave. I was then able to do it a second time with both champs taking damage.
after this champ is damaged by basic attack, cleave does not specify basic. it just says this champ makes a attack, you are right but i agree it should be classified as basic as cleave goes around most preventive measures.