[Bug]: Internal attendees do not receive calendar invitations (events not shown) while external email attendees work

⚠️ 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

Internal users do not see events when added as attendees
External email attendees receive invitations correctly
RSVP works for external email attendees
ATTENDEE data is correctly stored in event (PARTSTAT=NEEDS-ACTION, RSVP=TRUE)

Steps to reproduce

1. Create two internal users (testa, testb)
2. Create event as testa
3. Add testb as attendee (internal user selection)
4. Save event
5. Login as testb

Expected behavior

Event should appear in testb calendar
RSVP UI should be available

Nextcloud Server version

34

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.3

Web server

Apache (supported)

Database engine version

MariaDB

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

Upgraded to a MAJOR version (ex. 31 to 32)

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

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

Configuration report

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

Start by reproducing the issue on Nextcloud Server 34 with two internal users, then compare the stored attendee data with the calendar visibility and invitation paths. Done means the internal attendee sees the event and RSVP UI, while external email invitations and RSVP behavior continue to work.