Conflict with 301 redirection and www links

Hello,

I have an old website that redirects to a new website.

On the internet, there are old blogs with links to deep pages of the old website. With an htaccess file and 301 redirections, links should return to the new website pages easily.

Both old website and links on it was written with : http://www.oldwebsite.com
Both new website and links on it are written with : https://newwebsite.com

The problem is this:

1) oldwebsite.com/file/oldpage.html ➔ redirection ➔ newwebsite.com/newpage.html (OK it works)

2) http://www.oldwebsite.com/file/oldpage.html ➔ redirection ➔ newwebsite.comfileoldpage.html (ERROR, slash / disappears)

There is a problem ONLY with the case n°2 including www and specific page – and I don’t understand where it comes from because if I do:

3) http://www.oldwebsite.com ➔ redirection ➔ newwebsite.com (There it works !)

I also tried to do a mistake myself like this :

4) www.oldwebsite.com/file/FAKEpage.html ➔ redirection ➔ newwebsite.com/404.php (that's what I want here)

So I am pretty lost right now. With this / disappear ? How it is even possible ?

Any ideas, fellas ?

Maybe 3rd video here helps a bit https://www.blackhatworld.com/seo/get-expired-domains-301-redirects-seo.1107814/

Are you trying to make a wildcard redirect? I don’t quite get it.