Flatline

• WWizards of the Coast2023-10-13
  Official Ruling
  Flatline affects only creatures your opponents control at the time it resolves. Creatures they begin to control later in the turn won't change power and toughness.
• WWizards of the Coast2023-10-13
  Official Ruling
  Flatline overwrites any effects that set a creature's power and toughness. Any existing effects or counters that raise, lower, or switch a creature's power and/or toughness continue to apply to the creature's newly-set power and toughness.

Rulings via Scryfall