I believe the WYSIWYG field will add the <p> tags automatically like the WordPress editor. The code I gave you before is used to remove the generated <p> tags, so the result will be something like the issue you have.
It’s possible that you have a custom function that removes the <p> somewhere else. Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
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