TransformAttributeResolver adds instead of replacing values

[Kimmo Koivisto]

Hello,

I'm trying to replace existing attribute with values from another attribute using Transform.
I am using 3.2.3 SP/centos 7.9.

When using transform, destination attribute is not replaced as I understood from documentation but transformed value is added and multivalue attribute is created.
https://shibboleth.atlassian.net/wiki/spaces/SP3/pages/2065334474/TransformAttributeResolver

Example source attribute before transform:
schacPersonalUniqueID:  urn:schac:personalUniqueID:fi:FIC:123456-123X

Example dest attribute before transform:
nationalIdentificationNumber: 654321-321Y

My transform is:
<AttributeResolver type="Transform" source="schacPersonalUniqueID">
  <Regex match="^.*:(.*)$" dest="nationalIdentificationNumber">$1</Regex>
</AttributeResolver>

As I understood from the documentation, dest attribute should be replaced by transform.
Transform regex reads the proper value from source attribute but does not replace dest attribute.

Expected result (content is replaced with transform):
nationalIdentificationNumber:123456-123X

Result:
Transform adds new value and creates multivalue attribute:
nationalIdentificationNumber:123456-123X;654321-321Y

How to replace existing values using transform or is there other ways to handle this?
If not possible, is there any way one could convert multivalue attribute to single value to fix this?

Cheers,
Kimmo Koivisto