The iCloud CardDAV interface occasionally returns vCards with empty FN value (https://www.rfc-editor.org/rfc/rfc6350#section-6.2.1).
The behavior seems to occur only since a few months (noticed by customer reports of our CardDAV client).
here is an example vCard:
BEGIN:VCARD VERSION:3.0 FN: N:LastName;Firstname;;; UID:.vcf PRODID:-//Apple Inc.//iCloud Web Address Book 2304B11//EN REV:2023-01-03T17:21:11Z ORG:; TEL;TYPE=HOME:<phone_nr> END:VCARD
After a resynchronization of the contact data, the vCards with empty "FN" values seem to vary (other contacts are affected by that issue than before).
Everything seems to be fine on the iCloud website or in the iPhone contacts app.