Debugging .htaccess is hell, and RewriteRules in particular if they are not working as expected. But fear not! RewriteLog will help you out. \
Add this to your vhost's configuration:

RewriteLog "/tmp/rewrite.log"
RewriteLogLevel 9

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After that, restart your Apache httpd and tail the above logfile.

When you are done and all is well: remember to remove those entries again, or set the log level to 0, to switch off rewrite logging.