Not exactly a solution, but, here’s an another (kind of similar) discussions:
and
The gist being, if you link to a URL without the trailing slash or if the user types the URL without the trailing slash, it would still be served. With Pretty URLs feature, it’s not actually redirecting, instead it’s just modifying links in your HTML to remove .html extension. But since, you have not linked with the .html extension, it is not changing the HTML anymore as it’s already ‘pretty’.
That’s what I understood from that thread. So, if that’s what it is, you might have to update your links to add trailing slashes to them. This would mean, users who use your internal website navigation would only land on pages with the trailing slash, however, if someone removes the trailing slash and tries to access the page, it would still be visible.