They’re not emoji, they’re img elements. I get the same thing in Mail for Mac, even after clicking “Load Remote Content” and “Load Content Directly” (Monterey 12.6.1). It’s a result of using System Preferences > Network “Limit IP address Tracking”:
Limit IP address tracking by hiding your IP address from known trackers in Mail and Safari.
My hunch is they’re mistaken from tracking pixels because of their small height and width (20px).