Galleries just started showing the wrong images

I was about to change an image in a page that I had edited and published last week without problems and, to my surprise, the Classic Editor is now showing all the latest uploaded images in every gallery of every post. Even if the HTML editor only references one or two images, the Visual Editor will show lots and lots of them.

Thankfully, this only affects the Classic Editor and every post in the site looks normal, but now I have to use the modern editor for the changes and future updates, which isn’t very ideal.

I assume that I must have updated the plugin some time after my previous changes have been done. Are there any recent changes which could have broken how galleries show up in the editor?

Thanks.

The page I need help with: [log in to see the link]