After the latest update to the game, in the PS4 version at least, phantoms no longer swoop to attack after their initial attack. They begin to swoop, but only do so a small bit before they begin their rise again. This makes it impossible to obtain Phantom Membranes, therefore making it impossible to repair Elytra without the Mending enchantment.

More information can, and will be provided upon request, if required.

Thank you!