How do I prevent Apple Contacts from automatically changing all phone number labels to 'Telephone'?

As of a certain moment in time, Apple Contacts has changed all labels of all phone numbers of all my contacts into the same label, being 'Telephone' ('Telefoon' in Dutch). This causes a lot of issues being I don't know the difference between different phone numbers of a contact, my favourites are messed up because it randomly picks a phone number as they all are the same label, ....

Correcting the labels is only ok for a certain time but automatically they are changed again. Adding a test contact with a phone number labelled as 'mobile' and another phone number labelled as 'work' is within the day changed into 'telephone' again.

I also contacted the Apple Support Team who are investigating, but cannot find the cause, nor the solution.

[Re-Titled by Moderator]