I’m running a Multisite Subdirectory setup with the following structure.

Example.com/ (main site)
Example.com/en-us

I’m trying to achieve the following using a custom structure in the permalink settings:

Example.com/blog/postname
Example.com/en-us/blog/postname

It works for the main site however it gives a 404 on the /en-us/ site.

I’m also running the latest version.

I must be missing something? Any help is really appreciated.