Thanks for taking care of this report. And pointing me to #12516 which 
is duplicate indeed. I might have used the search page wrongly ...

This has been discussed in #12516 already.

Since Horde only exposes some of the values that are stored in the 
database via CardDAV, but not all of them, care needs to be taken not 
to unset values that cannot be set via CardDAV.

When a CardDAV client wants to change a property of an address book 
entry, it just sends the new property value. Along with all other 
properties of this entry, which _have a value_ (no matter if changed 
or not). This works well with Horde.

If the clients wants to _remove_ a property , the client just does not 
send it. Instead it sends all other properties of the entry, which 
_have a value_ (no matter if changed or not).

Horde does not remove this (not sent) property . It just keeps its 
former value.
That means it is impossible to remove property values via CardDAV.
Removing properties via Horde web frontend or ActiveSync works as expected.

This misbeavior is coming from function davPutObject($collection, 
$object, $data)
in turba/lib/Application.php
https://github.com/horde/turba/blob/master/lib/Application.php#L1107
Horde only changes properties sent by CardDAV client , unsent 
properties just stay within the former existing object, which then get 
stored to database.

My suggestion for improvement:

Putting these lines before foreach ($contact as $attribute => $value) 
{ (between line 1104 and line 1105)

                 // Set every property to null (except those starting 
with '__')
                 foreach ($existing_contact->attributes as $attribute 
=> $value) {
                     if ( strpos($attribute, '__') !== 0 ) {
                         $existing_contact->setValue($attribute, null );
                     }
                 }
                 // tags/categories should not be null, but empty string
                 $existing_contact->setValue( '__tags' , '' );

Looking forward to seeing this fixed.