Hi,
Kindly check your WordPress editor for bold and h3 tags. This could happen if a closing bold code is missing in the editor. Better to start fresh by copying to text file and empty the area and paste.
To check: Copy the whole content to simple text file, create a test post and paste from the text file in the html view. You cannot use Word for copy pasting as it comes with its own formatting. This is not a theme related issue.
You can also choose to use the https://wordpress.org/plugins/classic-editor/ if you are having difficulty with new editor.