Niantic is currently investigating the issue in Pokémon GO where Primal Groudon is missing 2HP when a Trainer uses a potion or max revive to fully heal it

Niantic has updated the official Known Issues support page for Pokémon GO. Read on below to learn more:

Primal Groudon does not reach maximum health.

Last Updated: 2h
Last updated: 2/24/23

Primal Groudon does not reach maximum health.
Issue description: Primal Groudon is missing 2HP when a Trainer uses a potion or max revive to fully heal it.
Issue status: Investigating

Source: Official Pokémon GO support page