/node/{id} not working for 404 exclusion

When our 404 gets hit (by bots), those are getting cached as they have multiple different test URLs.
So we get TONS of cache files for nothing.

Our 404 page is set to /node/5871 in /admin/config/system/site-information

I tried entering the 404 node ID within the "Pages to exclude" field ( /node/5871 ), the pages are still getting cached.

So, to test I created a specific content type for the 404 page and excluded the whole content type, this works and does not create cache files.

Is there any way of making this work without creating a specific content type?
Why the /node/{id} does not work for the 404 page in the "Pages to exclude" field?

Thanks!