This exact thing happened with a casual dining chain I worked with in Toronto.
First 5-7 days looked solid. Tight targeting, decent CTR. Then around day 8, delivery silently shifted into 55+ audiences even though we'd set 21-40. Engagement stayed flat. But dine-in reservations? They tanked. Went from around 40 bookings a week down to less than 20. CTR was still hovering near 4%. Makes you think Meta's working fine. It's not.
What's happening is "audience expansion" creep. Meta respects your filters early, then once it collects proxy signals (clicks, messages, directions requests) it starts chasing cheaper "likely converters". For restaurants those proxies are garbage. Someone clicks a menu link because they're bored, not because they're booking a table. Meta optimises for that engagement and drifts into older, less valuable audiences.
We rebuilt the campaign around reservation confirmations only. Fed actual booking data back through CAPI. Audience drift dropped massively. Within a month we were back to 50+ bookings a week and cost per reservation fell from $19 to $11. Meta finally started chasing verified diners instead of click-happy randoms.
When you check your campaigns, are those older audiences actually generating real reservations? Or just engagement with no footfall?