I am having the same issue. If WYSIWYG field is used in Gutenberg with position set to ‘Side’, WYSIWYG field is editable at first, but if you select any blocks in the page changing the Gutenberg sidebar tab to ‘Block’ instead of ‘Page’ (taking the WYSIWYG field out of view), and then go back to the ‘Page’ sidebar tab, the WYSIWYG Visual tab is not editable and any content in the Visual tab is gone. However the ‘Text’ WYSIWYG tab does contain its content.
I verified not a plugin or theme conflict by using Twenty Twenty One theme with all plugins deactivated except ACF Pro.
