There's got to be some earlier rule intercepting requests for /evenementen/ so by the time it reaches the RewriteRule, it's in a different form and the Rule can't touch it.
it is a Wordpress site so those URLs are already the result of a rewrite
Rewrite or redirect? A Rewrite doesn't change the URL; it can only change the source of the content.
Wordpress or not, the redirects (external) have to come before the rewrites (internal). If you have boilerplate htaccess that came with the Wordpress installation, there is probably a lot of extraneous stuff.
:: contemplating writing up another piece of boilerplate on "how to change your out-of-the-box htaccess into your own personal htaccess" :: Tangential: I only recently (like, say, last week) figured out why people keep getting the "rewrite" concept backward. In a rewrite, you're creating a discontinuity between a page's visible name and the source of its content. Everyone gets that part. The part they get wrong is this: The name stays the same while the content source changes. Not the other way around.