ACF Fields in Media Editor popup behaves weird when Smush is enabled.

Install ACF plugin. When installed create a new Field Group. You can call it Media Test and set Location to "Attachment" is equal to "All".
Then add a wysiwyg field to this field group and call it Test Editor with Tabs to "Visual Only" and Toolbar to "Basic".
Then edit a post and click the "Add Media" button and pick any image in your media library.
If Smush is active on the site you should see the Test Editor custom field and it should be rapidly toggling between the wysiwyg editor and simple text editor.