[easyThrowaway]

By the way, the issue was cunningly appropriate for this thread: Considering that in wordpress EVERYTHING is stored in the DB as an article in the WP_POST table (Yes, even stuff like attachments, images, and the menus) an hook from a slider plugin was messing up with the image descriptions, which are stored as...the main body text of an article. Which is actually an image.

In a classic wordpress install this would just cause some weird garbage in the output, but given that in a gutenberg+ACF setup the content data is passed to the React/Block rendering engine, it would absolutely go crazy.