Yoast SEO is changing my slug!
This should no longer occur when you are using the most recent version of Yoast SEO. This used to be a feature called clean slugs which automatically removed stop words from the generated slug. It has always been bug prone and is not really important for SEO anymore, so we removed it in Yoast SEO 7.0.
What is this "slug" thing you talk of?
The slug is the part of the URL that distinguishes the post or page, so in this URL
http://example.com/hello-world/ the slug is