Proactive web messaging to previous guest returns 404

Trying to send a proactive notification to a guest who had a session yesterday using POST /api/v2/webmessaging/organizations/{orgId}/guests/{guestId}/messages. The endpoint returns a 404 Not Found even though I can see the guest record in the admin UI.

{
 "type": "text",
 "content": "Are you still there?"
}

Is the guest ID invalid after the session closes or am I hitting the wrong endpoint?