PATCH /conversations/chats/{chatId}/participants stuck on sending canned response

Trying to push a canned response to the customer mid-chat via the Conversations API. The PATCH request hits /api/v2/conversations/chats/{chatId}/participants/{participantId} with a 200 OK, but the agent sees the message while the customer gets nothing. Payload below:

{
 "send_to": ["customer"],
 "text": "Here is the info you requested."
}

Checked the conversation log and the message is marked as system-only. What am I missing?