What you are seeing is correct and apparently intentional from Monday.com:
Can I share folders?
Due to security reasons, it is currently not possible to share an entire folder (we wouldn’t want you to share a board by mistake that was in that folder). That being said, we have a work around: if your folder is within the Shareable boards section, you can always share all of the boards within it.
The only workaround I can think of is if you organize your projects into Workspaces instead of folders. But depending on how many projects you have, this may not be feasible. Without knowing much about your board structures, you might be able to create each client a dashboard instead. All the relevant information would then be viewable to them and you can customize what they see.
Let me know if you have any questions regarding Workspaces or Dashboards. Hope this helps!