Existing iPhone contacts don't sync to NextCloud contacts app

Good Day!

I solved many issues with my NC instance myself through the help of this forum but I was not able to find a solution for the following problem:

My existing iPhone contacts don’t sync to NextCloud.

When I create a new contact in NC it syncs immediately to my iPhone.
When I update the contact on the iPhone it syncs the change to NC.

When I create a new contact on my iPhone it syncs immediately to NC.
When I update the contact on NC it syncs it immediately back to the iPhone.

How can I trigger a full sync of my iPhone contacts to NC?

I tried turning off iPhone to iClould contact sync. I haven’t yet rebooted my phone. Do I maybe need an app specific password? Don’t think so as I don’t try to sync NC with iCloud. Out of ideas…

TIA!
Olaf