If I have GPG support out and it brings in a creature from my graveyard that I already have on the battlefield it doesn’t get exiled at the end of my turn and I keep the reinforcement.
Yup! Its always been like that. If you cast a creature that was just returned with GPG all reinforcements get exiled at the end of the turn.
Its almost definitely wrong, but with how long its been I feel weird calling it a bug.