Like what Satomine Night said above, I'll say the same. It's not confirmed whether Poipole willingly left Ultra Space or not. It could've been easily sucked into the Ultra Hole and as a result ended up being transported to this world. Yes it does enjoy Alolan atmosphere, which is full of light(to which it's very much attracted) doesn't anyhow mean that it doesn't want to go back to its own world that is Ultra Space, to where it originally belongs. So your logic of Poipole willingly leaving Ultra Space doesn't hold much water here because it's not confirmed by any means.
Anyway, the anime has shown a clear trend in this kind of circumstances where a Pokemon of Ash finds its own home/family, to where it actually belongs, that in this cases the Pokemon ends up returning to their own home/family and Ash releasing them, regardless of how strong a bond they share with Ash. This is clearly proven by the Lapras/Goodra cases, where despite both of them sharing a strong bond with Ash, they ended up staying back with their family/at their homeland, and in the process got released. That very likely to happen with Poipole as well, given the foreshadowing by Lusamine's words in its capture episode. Saying that Poipole can remain staying with Ash because of the close connection/bonding it'd share with Ash is a pretty weak arguement to use, because as evident from the Lapras/Goodra cases, this is not the trend the anime follows in this cases.