Beats me, I can’t replicate this issue.
The issue being…
…once I choose a Custom Header in the Layout Options box while editing a page, the selected option is not saved. After selection and updating the page, the dropbown is again at – Select -.
With a fresh install of the Mingle theme I can’t replicate that issue.
Have you tried reinstalling the theme? Use FTP to delete the existing Mingle theme folder on your server and then upload the new theme folder from your computer to your server. It’s important that you first delete the existing Mingle theme folder on your server before uploading the new one.