Redirection breaking for posts with modified URLs

Hi,

I’ve noticed that quite a few pages on my site are hitting 404s. It looks like this is happening when a post URL is edited, e.g.:

/2017/06/how-to-fix-my-car
changes to
/2017/06/fix-car

Now, my understanding was that WordPress is supposed to automatically track post URL history, and if the URL is changed, automatically redirect to the new one – so that you can clean up verbose or incorrect URLs without prejudice. It doesn’t appear to be happening though.

I installed the Redirection plugin (https://en-gb.wordpress.org/plugins/redirection/) a few months ago to add some ‘short URL’ style redirects in for tersley deep-linking to the site – is it possible that the plugin is breaking the automatic tracking of modified URLs? Currently I am using its 404 log tool to add redirections manually to fix these issues, but it feels like I shouldn’t have to do this.

Any idea what might have happened, and how to fix it?

And while we’re at it, are there any good tools that will report broken links? I tried the Broken Link Checker plugin, but it didn’t report any broken links – despite the fact that there were internal 404s that have been found, logged (and subsequently fixed) via the Redirection plugin.