WP Version: 5.1.1
ACF Pro Version: 5.7.13
Steps to recreate:
- Create an ACF image field to appear on pages
- Add a new page (doesn’t need to be saved)
- Add an image using the custom field
- Click the pencil icon to edit the image
- Click “Edit Image”
- Either edit the image or don’t, doesn’t matter
- Click “Back”
- The “Update” button is now blank and grey instead of saying “Update” and being blue
This happens with both Classic Editor and Gutenberg
(Screenshot below)

This could be something in WP core, but I’m not sure. You should submit this but here https://www.advancedcustomfields.com/contact/