Backup Contacts does not include categories

[bohni]

⚠️ Before posting ⚠️

• [x] This is a bug, not a question or an enhancement.
• [x] I've searched for similar issues and didn't find a duplicate.
• [x] I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
• [x] I agree to follow Nextcloud's Code of Conduct.

Steps to reproduce

1. Create a contact with two contact groups "category1" and "cat2"
2. Enable backup of contacts
3. search for contacts vcard in a backup *.vcf file in .Contacts-Backup
4. The contacts vcard doesn't include contact groups

I use Thundebirds CardBook AddOn to sync with nextcloud. Exporting the contact gives me a line CATEGORIES:category1, cat2

Exporting the contact via Nextcloud Contacts Web App (download) includes the CATEGORIES line, too

Expected behaviour

Contact groups should also be included when backing up contacts

Actual behaviour

The vcard misses the CATEGORIES:... line

Android version

16

Device brand and model

Pixel 8

Stock or custom OS?

Stock

Nextcloud android app version

3.33.0.RC3

Nextcloud server version

31.0.9

Using a reverse proxy?

No

Android logs

No response

Server error logs

Additional information

I think, creating a vcard entry while creating the backup does not log single fields that are exported.