Someone claims that it maybe because iOS15 would no longer recognizes Lets Encrypt certificates.  Could this be the reason why the images are not showing?

Hi, I have exact same problem. Did you find a solution? Since iOS15, Apple's native Mail app shows our organization's email images as alt text instead of the image. We use Wordpress and send email when new blog post are publish. Our emails work fine in ALL other mail clients, but for some reason Mail blocks them, and does not present an option to download remote images. BTW, my iOS Mail Privacy Settings are OK. Other emails come across fine and images are shown.