Slash added to .htaccess redirects, so .htaccess not working

You have a mismatch between the URL format in your settings, and then in .htaccess.

If using WP to redirect, you save urls with a preceding forward slash, e.g. /page-to-redirect

If I want to save URLs in .htaccess, or download the .htaccess settings in your plugin, you keep this slash in there, so you end up with:
RewriteRule ^/page-to-redirect$ /new-page/ [R=301,L]

This is wrong, this rule is not working in .htaccess, you need to remove the slash. You do for my .htaccess anyway, which is a Centos server with Apache.
RewriteRule ^page-to-redirect$ /new-page/ [R=301,L]

So I cannot use .htaccess, your plugin is not saving it correct in there. I prefer to use .htaccess otherwise you cant redirect things like media uploads, e.g. a PDF

Also, if I edited a URL in your plugin it was not saving the change in .htaccess either.