I'm not sure where else to look. As mentioned this was working fine with appsettings as per Umbraco Documentation until I installed SEO Checker package.

I have the ErrorController also as per the Umbraco Documentation and that still works for 500s. Doesn't do anything for 404s normally, and I tested adding that statusCode to it but same thing: default Umbraco 404 page.

Any ideas on where to look for what I've done that could be interfering? There's no custom route hijacking in the project. The only custom stuff I've written is a couple html field helpers.

I only have Contentment, uSync, & Node Defender installed otherwise for packages. I've tried cleaning and rebuilding and closing, even did a nuget cache clear, etc, to see if there's anything hung up.

Content Tree wise it's:

Site Definition (root) -> Home (w/ hostname) -> Folder-w-error-pages -> Page404

Gunna try removing package and see if problem persists. That's the only other thing I can think of to try. thinks Just to at least make sure I haven't done anything otherwise to mess it up by itself :3

Who knows!? Maybe a reinstall is just the magic it needs.

Edit: Confirmed everything works fine again after removing SEO Checker. Get custom 404 page instead of default Umbraco. Appsettings piece is just this:

"Error404Collection": [
  {
    "Culture": "default",
    "ContentKey": "3f1a622b-90f3-4bf4-8f3b-9c50a36db221"
  }
]

Which as mentioned in documentation isn't expected to work with SEO Checker. Reinstalled SEO Checker and back to same problem. Always default Umbraco 404.

Edit 2: Bonus test. Tested 404 setting with selected page that has SEO Checker properties and one that doesn't (page404 doesn't). No effect. Long shot but eh, worth a try xD

I have checked the Umbraco logs but there's nothing around it.

Also have tested this with CSP disabled & toggled Umbraco's default URL Tracker on/off but no effect in any case.