I have a multisite network, and my theme has a custom `404.php`. On all the child sites this 404 handler is being correctly used, but on the primary site all 404s go back to the home page without touching my `404.php` at all.

And for once, this isn't the fault of the Redirection plugin! The problem is still there when Redirection is turned off (network-wide *and* on the specific site).

The problem isn't occurring on my local copy of the same site, but I can't find any difference in the configuration or plugins that could be causing it.