1) try setting the 'on change microflow' or 'on key press' of the CKEditor to a new microflow that refreshes the context entity. This may cause things toe be slightly clunky so you could also try:
2) adding a boolean value to your context entity called textEntered, then you can have your validation microflow check to see if the string you edit is empty or not. If it is empty, use conditional visibility display on the page. Then you can set the 'on key press' microflow to change the boolean to true. This will give you the ability to put the validation text above the CKEditor in the page, which might look a little better.