BVN 404 handler not triggered for invalid deep URLs

I use the BVN 404 handler on a website with both CMS and Commerce content.

However, it only triggers on URLs to non-existent pages directly below the start page. It does not trigger on URLs pointing to non-existent pages deeper into the site tree.

For instance:

• /not-existing: redirected to custom 404 page.
• /not-existing/not-existing: not redirected
• /existing-page/not-existing: not redirected (when existing-page is an existing page)
• /ar/not-existing: not redirected (ar is a language code)
• /catalog/product/not-existing: not redirected (when both catalog and product exists in Commerce)

For the URLs not being redirected, I receive a 404 response with no content (Content-Length: 0), looking like no custom error handling took place.

Obviously the customer wants to use the same custom 404 page for all of the above cases.

Any advice on fixing this?