On Autobattle, Selena's Ability 3 ( removing debuff) is being used by autobattle mode only when your heroes need healing. This is how Autobattle AI works, we aware of that and thinking about changing the algorithm. Meanwhile, we recommend not to use autobattle when you use Selena and if you want the Autobattle AI to automatically choose Ability 3.

When Selena uses Ability 3 and applies a buff, and after that applies the same buff using her passive ability, the icon gets stuck and doesn't dissappear.

When Selena uses Ability 3 , it doesn't remove Acorn that is applied by Wizz & Ard.

If  all allies have active Buff Immunity, then if Selena's passive ability is applied,  her passive ability doesn't remove the enemies' buffs.