If when a page loads a user starts on the text tab of a WYSIWYG (either the post content or a meta box) and clicks the visual tab - although the UI looks to have changed - the content still reads from the text tab.
This means clicking the text tab does nothing as it thinks it is on that tab already.
Entering any content in the visual tab does not save - as the saved value is taken from the text tab.
Clicking the visual tab a second time loads the tab properly.
This is a classic editor post (regards less of if the post type supports the body), Metabox 5.4, WordPress 5.7 and a clean install with no other plugins.