From the 12th of October Facebook is removing their 28-day attribution model. All accounts will revert to 7-day attribution by default.
Facebook sent this announcement directly to advertisers via email. Announcing that upcoming privacy initiatives are limiting the ability to measure interactions across domains and devices. This includes shortening the period of time to attribute conversion events.
Facebook believes that this will make their advertising more resilient to future privacy changes. The 28-day attribution data will be available until October 12th. After that it will be removed, so if you need any of your historical data you should download it as soon as possible.