Containment Priest is exiling creatures that enter the battlefield at the same time it does

[BOOMcamel]

Twice I've exiled [[Containment Priest]] with another creature using [[Lagrella, the Magpie]], and the priest exiled the other creature upon reentry both times. One time it was a pair of priests, which was interesting because only one of them got exiled upon reentry and the other remained on the battlefield.

[github-actions]

Containment Priest - (Gatherer) (Scryfall) (EDHREC)

{1}{W}
Creature — Human Cleric
2/2
Flash
If a nontoken creature would enter and it wasn't cast, exile it instead.

Lagrella, the Magpie - (Gatherer) (Scryfall) (EDHREC)

{G}{W}{U}
Legendary Creature — Human Soldier
2/3
When Lagrella enters, exile any number of other target creatures controlled by different players until Lagrella leaves the battlefield. When an exiled card enters under your control this way, put two +1/+1 counters on it.