[Bug]: Federated group share is not removed when deleted from remote

⚠️ 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

With federated group shares (so shares not shared with a user, but with a whole group), when the remote share no longer exists it's not automatically deleted from the target server.

That leads to the share being recreated for every user that's in the group on every start of the files app, showing a request "Do you want to add the remote share" on every reload.

Steps to reproduce

1. Share a folder from server A via federation with a group on server B
2. Delete the shared folder from server A
3. On the next reload, server B will try to recreate the share for all users of the group, because the actual group share is not removed.

Expected behavior

Server B also removes the entry for the group share, not just the entries for each user share.

Installation method

None

Nextcloud Server version

27

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by tracing federated group-share handling in the files app, focusing on cleanup when a remote share is deleted. Reproduce the three steps with a group share and inspect the behavior on reload. Done means the group-share entry is removed on the target server along with its per-user shares, so the share is not recreated or requested again.