Exile multiple creatures with Parallax Wave, including Containment Priest.
Parallax Wave then leaves the battlefield.

Actual result: Containment Priest returns, as does a creature that was Wave'd after Containment Priest had been Wave'd. The two creatures that had been Wave'd before Priest was Wave'd are permanently exiled.

Expected result: all creatures exiled by Parallax Wave return (as Priest does not affect itself and cannot affect creatures entering simultaneously with it). (I confirmed this expected result with Eli Shiffrin.)

(Perhaps Parallax Wave is incorrectly returning creatures reverse-sequentially instead of simultaneously?)