
There could be several reasons why a user who has been added to the site owner group is experiencing issues with a SharePoint 2019 on-premises site. Some possible causes include:
- Caching issues: The user's browser may be caching an old version of the site, preventing them from seeing the updated content or permissions. Clearing the browser's cache or accessing the site in an incognito/private window may resolve this issue.
- Browser compatibility: The user may be using a browser that is not fully compatible with SharePoint 2019. Updating to a more recent version of the browser or using a different browser may resolve this issue.
- Permission inheritance: The user may not have permissions inherited from parent sites. In this case, you can check the permission inheritance of the site and make sure it's set to inherit from the parent site.
- Site Collection administrator: The user might not be added as site collection administrator, and you should check if the user is added as a site collection administrator.
It's important to check all of these possibilities and troubleshoot accordingly. If the issue persist, it might be helpful to check the SharePoint logs for more details on the issue.