Here’s a little baffling problem: after developing a new site locally and pushing the files to the live server the layout was suddenly broken.
After determining neither the theme nor plugins were to blame I noticed that the WordPress core CSS files for the blocks were missing. They never got loaded or even requested.
After digging into some comment threads I found this solution on the WP forums:
Delete the _transient_wp_core_block_css_files entry in the wp_options table of your database.
Note that this is only an issue if you use a theme that needs the core block CSS to work, otherwise you’re good. 😀