How to set up read-only and sync // "Not allowed because you don't have permission to add files in that folder"

Nextcloud version : 21.0.3
Operating system and version (eg, Ubuntu 20.04): hosted at Hetzner
Apache or nginx version (eg, Apache 2.4.25): replace me
PHP version (eg, 7.4): replace me

The issue you are facing:

I created a folder owned by admin, and shared with users. one user has full permissions, and there is no problem syncing the folder to his Mac.
The other user has read-only permissions. On her Mac the desktop client displays error messages “Not allowed because you don’t have permission to add files in that folder”. However, the folders and files seem to be sync onto the Mac fine.

How do we set up the permissions for a folder that one user can read-only and sync down to their desktop?