Thanks for the quick reply!
It’s the Gutenberg editor, but I’m actually talking about new posts, not old ones.
We usually write programming-related posts in Markdown and then copy-paste them to the Gutenberg editor. Multiline-code snippets (3 back-ticks) are rendered correctly as Enlighter code block, but inline code (1 back-tick) is not. It’s always rendered as the default WordPress inline code element.