The POWR Email Countdown works by generating and displaying a repeating gif of the timer. The gif is generated when the viewer attempts to load the image from the POWR servers.
For most users, this happens at the time that the recipient opens the email, and this is what enables the Email Countdown to reflect the correct time until the end date.
With iOS 15 Mail Privacy Protection enabled, images are downloaded ahead of time to Apple's private servers. The Email Countdown will still be generated and display in the recipient's email, however the time will correspond with when Apple downloaded the image, not necessarily when the recipient opens the email.
For most users, this will not be a significant difference. 23% of emails are opened within the first hour after delivery, and many more opened shortly after. However, keep in mind that recipients with these privacy settings enabled may see a version of the countdown with more time than actually remains.