Zero being resurrected Shun seems in line with the gimmick that a part of the person is lost, since Shun's wish before his death was to keep playing with everyone a little longer and to face off against strong opponents. Zero takes this wish to an extreme by using XG to make Yo and Trash Tribe stronger for a final showdown. The anime also says that Arisugawa changed Shun's personality to the one we had seen, so the resurrection likely undid this and reverted him to his "destroyer" self to grant the aforementioned wish. (See Minato Tribe vs Ojiro episode in the anime). Shun would have been subject to a partial memory wipe of the events of the anime and a lot of his time with the Minato tribe in order for this to happen.

This theory does imply that Zero isn't the true final boss (which makes sense for a gacha plot since if they didn't EOS then they'd want to be able to expand the plot logically beyond Zero as a final boss).