Since the update to Nextcloud 19.0.4 and the subsequent update of the Contact App 3.4.0 and 3.4.1, field names can no longer be changed.
After entering an individual text as field label,
the field has disappeared and you have to insert the field e.g. โPhoneโ again and fill it with values again.
Can you please be more precise and explain IN DETAIL how it looked like and how it is currently looking. Screenshots e.g. can make it easier to understand your problem. If possible please also provide a step-by-step description how the problem can be replicated.
Sorry for the late message, but the problem still exists with the Nextcloud version 20.0.4 and with the Kontakt App version 3.4.2.
In the contacts the labels for phone, email and address can be edited, but after the confirmation the whole field including its label disappears and you have to insert it as a field from the new properties back into the form!