Possible bug? XZ immune to damage from post-summon Tibbers while Annie outside circle

Reposted from Help & Support at Broporo's suggestion to get clarification.

Is it intended that Xin Zhao be immune to damage from Tibbers' persistent AOE damage and autoattacks while Annie is outside the circle of his ult? It's one thing for him to be immune to the initial Tibbers summon damage if Annie is outside the circle when she summons him (where Annie could definitely be argued to be the "source" of the damage), but once Tibbers is summoned, aside from tower aggro considerations, Tibbers could in some ways be considered the "source" of his own AOE damage and autoattacks.

Is this intended, an oversight, or a result of somehow using the tower aggro/death recap system (which is presumably there so an Annie can't towerdive solo without taking aggro on herself) to calculate where a damage "source" is?

EDIT/UPDATE 2017/10/30: It seems like if Annie dies while inside the circle, Tibbers can continue to damage Xin even if he moves so her corpse is no longer inside the circle. If Annie dies while outside the circle, Tibbers continues to NOT do damage, even if he moves so her corpse is inside the circle. Definitely not consistent with expectations.