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

### ⚠️ This issue respects the following points: ⚠️ - [x] This is a **bug**, not a question or a configuration/webserver/proxy issue. - [x] This issue is **not** already reported on [Github](https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3Abug) OR [Nextcloud Community Forum](https://help.nextcloud.com/) _(I've searched it)_. - [x] Nextcloud Server **is** up to date. See [Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) for supported versions. - [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/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? - [x] Default user-backend _(database)_ - [ ] LDAP/ Active Directory - [ ] SSO - SAML - [ ] Other ### Configuration report ```json ``` ### List of activated Apps ```shell ``` ### Nextcloud Signing status ```shell ``` ### Nextcloud Logs ```json ``` ### Additional info _No response_