The site had just learned to draw its own preview images, and they were good pictures - except that nobody could see them. Every address it wrote into a page pointed at the domain the project intends to live at one day, and that domain still answers with an older site. So a link to a post arrived in a chat as a broken picture or as the wrong one entirely.
The addresses are built from the place the site is actually served from now, and from the intended domain only when there is nothing better to go on. The preview images, the canonical addresses, the sitemap and the structured data all follow the same rule, so a page describes itself at the address where it can actually be reached.