Mutate with Kami of Mourning
Warning, this is a complex rules issue and perpetually isn't in the comprehensive rules.
When a creature is mutated, it takes on the quality of the top most card, including mana value. Previous ruling on card Death's Oasis allows you to bring back any card that was mutated on as long as mana value was less than the top most card. This is where Kami of Mourning isn't functioning as Previous rulings. In Mtg Arena, when the perpetual effect is applied, then the creature dies, it does NOT return any cards in the mutated pile when it should. (Dies is short for when a creature is put into the graveyard from the battlefield so the card is in the graveyard prior to death trigger).
Also, perpetually isn't applying perpetual effects to each card involved in the mutated creature, only the original creature. This conflicts with other rulings such as Undying returning all cards on the mutated creature, same effect as Undying Malice.