Editing existing contact in Nextcloud generates double entries on iPhone

If I edit an existing contact in Nextcloud, it generates a new contact in my synced iPhone contacts (and only on iPhone, not in Nextcloud itself) instead of updating the existing one.
If I edit it again in Nextcloud, the newly generated contact on iPhone gets updated - as it should be.

Did you ever hear about this or do you know how to solve this?

Nextcloud 21.0.1
PHP 7.3.27
Contacts 3.5.1
IOS 14.5