During the 2021 Apple’s Worldwide Developer Conference, Apple announced the new iOS 15 which allows users to utilize the new Mail Privacy Protection feature. One feature within this privacy update, blocks trackers that lets email senders know when the recipient opens an email.
With this update, the Studio Admin Dashboard cannot report on email open rates for Apple Mail users. Due to this the insights within the Studio Admin site will likely reflect a drop in open rate effectiveness.
This does not mean customers are not receiving or reading the emails, it simply means due to the increased privacy protection from Apple the open rates can no longer be tracked. Other email metrics can still be tracked including clicks, and non-Apple users.
For more information on Apple's release: