After editing a contact VALUE=UNKNOWN is written in the vcard-file

Describe the bug

Contacts are synced from phone to nextcloud. Everything is fine.

vcard:
BEGIN:VCARD
VERSION:3.0
PRODID:-//BusyApps//BusyContacts 2023.2.1//DE
FN:Vorname Name
N:Name;Vorname;;;
EMAIL;TYPE=HOME;TYPE=INTERNET;TYPE=pref:name.vorname@tld.de
TEL;TYPE=CELL;TYPE=pref:+49 123 4567890
REV:2023-08-14T15:55:40Z
UID:70D5E2AD-9D94-4D9F-9828-XXXXXXXX
X-BUSYMAC-MODIFIED-BY:
END:VCARD

Contact is edited in Nextcloud.

Steps to reproduce

1. Go to Contacts App
2. Edit a contact
3. download the vcard file
4. Open File with editor

Expected behavior

vcard file makes the changes, e.g. Mailadress or phone number

Actual behavior

vcard has the entries "VALUE=UNKNOWN" within the phone fields

vcard:
BEGIN:VCARD
VERSION:3.0
PRODID:-//BusyApps//BusyContacts 2023.2.1//DE
FN:Vorname Name
N:Name;Vorname;;;
EMAIL;TYPE=HOME;TYPE=INTERNET;TYPE=pref:name.vorname@tld.de
TEL;TYPE=CELL;TYPE=pref;VALUE=UNKNOWN:+49 123 4567890
TEL;TYPE=HOME;VALUE=UNKNOWN: +49 6353 373839
REV:2023-08-14T15:55:40Z
UID:70D5E2AD-9D94-4D9F-9828-XXXXXXXX
X-BUSYMAC-MODIFIED-BY:
END:VCARD

Contact version

5.3.2

Operating system

NC 26.05/26.04 Enterprise (on both)

PHP engine version

None

Web server

None

Database

None

Additional info

No response

Start by reproducing the edit in the Contacts app and downloading the resulting vCard, comparing it with the reported examples. Trace the vCard generation path in the contacts repository; done means edited fields are preserved without unexpected VALUE=UNKNOWN parameters.