POST /v1/subscribers with X-Buttondown-Collision-Behavior: overwrite no longer returns a 201 while leaving an unsubscribed subscriber unchanged. Overwrite cannot change terminal types (unsubscribed, blocked, complained, undeliverable) and returns 400 with subscriber_suppressed instead.
To resubscribe, use X-Buttondown-Collision-Behavior: add (which still flips unsubscribed → regular) or PATCH with {"type": "regular"}.
See Creating a subscriber for the full add vs overwrite distinction.