I seem to have the same problem. Is this a bug? Seems a very common use case to me.
My example:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
# Created by Redirection
# Tue, 05 Jul 2016 08:44:25 +0000
# Redirection 2.4.5 - http://urbangiraffe.com/plugins/redirection/
<IfModule mod_rewrite.c>
RewriteRule ^/imprint\.html$ http://domain.tld/impressum/ [R=301,L]
</IfModule>
# End of Redirection