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Hi,
In 301 Redirects either you set a custom URL to redirect to (in which case I assume you would have noticed if you had wrote it with http://), or you pick a Page, Post etc. in which case 301 Redirects uses the built-in WordPress get_permalink() function. In that case other plugins could affect or filter the URL and change the prefix. I don’t know how you could find out which unless you disable other plugins and enable them one by one to see which could be causing it 🙁