On WP 5.5 (but only in few cases) the Newsletter drag and drop editor was saving the emoji as images. Actually was not a problem of our editor, but a common behavior of WP replacing the emoji with SVG or PNG images on admin pages.
On release 6.8.6 we disabled that behavior ONLY on our admin pages. This is what even Gutenberg and WooCommerce do in their pages, being it a common problem.
So, if you’re experiencing the conversion of emoji to images in your newsletter, please update now.