In the HTML editor window for the post, I enter a quicktag <!--more-->. Then, press "Publish" or "Update" to push the change live.
<!--more--> immediately changes to & lt;!--more--& gt; (I added spaces here between & and lt/gt to avoid automatic conversion in the post) in the editor window, and the post doesn't have a correct "more..." link, but rather a <!--more--> text in its body.
I searched the documentation carefully and there's no answer on how to fix this issue. The environment was refreshed recently and I assume there are no changes in the_content() or the_excerpt() functions in the WordPress.