[Bug]: Federated Cloud Sharing from Public Link Fails

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When attempting to add a public shared file to a user's Nextcloud instance via the "Add to your Nextcloud" button, the process fails. This occurs when using the Federated Cloud Sharing.

Steps to reproduce

1. Create a file, then create a public share.
2. Copy the Federated Cloud ID from /settings/user/sharing.
3. Open the public share in a different browser or a private browsing session.
4. Click "Add to your Nextcloud" in the top-right corner.
5. A warning dialog appears

"Error while adding the public link to your Nextcloud"
Warning box:
Sharing of "TestShare" failed because [exampleUser1@examplecloud.com] was not found. The server may be unreachable or is using a self-signed certificate.

Expected behavior

A prompt should allow the user to add the file or folder to their own Nextcloud instance via a federated share, and the process should complete successfully.

Nextcloud Server version

30.0.8.1

Start by reproducing the failure from a public share using the Federated Cloud ID from /settings/user/sharing, then follow the “Add to your Nextcloud” flow and its warning dialog. Trace the federated sharing request from the public-share entry point to identify why the remote user is reported as not found. Done means a public file or folder can be added successfully to another Nextcloud instance through Federated Cloud Sharing.