Facebook Comment-to-DM: Sent is high, Delivered is around 3 to 4 percent, no error shown

My Facebook Comments automation (keyword trigger, Send Message set to "As private reply") has worked reliably for weeks. Now most new subscribers aren't receiving the automated private reply, even though the trigger fires correctly and they're added as contacts. Example: one contact commented, was correctly opted in through the Facebook Comments widget, but no message ever appears in her chat history. My flow shows 81 Sent but only 3.7% Delivered. I've already ruled out: wrong Send Message setting, expired permissions, other connected apps taking thread control (checked and disabled), Facebook's native Instant Reply (confirmed it doesn't apply to comments), and message volume (well under Meta's rate limits). Support ticket open since Wednesday, no response yet. Has anyone else seen this recently, and did you find a fix?