Gutenberg block previews suddenly not working

I have an issue with the Gutenberg block previews suddenly not showing at all when hovering over available blocks in the side panel. I have tried:

  • Commenting out custom code in the functions.php, including removing any enqueued JavaScript and CSS from the admin
  • Deactivating plugins (such as Advanced Custom Fields)

But the preview popup is still not showing. It worked initially, but right now it’s not working at all. I can’t see any errors in the JavaScript console or by enabling debug in the wp-config.php.

The expected behaviour would be for each block to show a preview and a description of the block that you’re hovering, but that popup is not showing as it is now. See this image for how it should look like:

enter image description here

I’ve tried searching for this on Google, but I can’t find anyone else having this problem. Has anyone here had this problem and knows what was causing it? Or is it a problem occurring because of a bug in WordPress itself?