Data element "default value" in Mobile Core Modify Data rule is not interpreted

Hi,

I'm capturing a CollectPii call from the (AEP) Mobile SDK in Experience Platform Data Collection. In the rule I use a Modify Data action to transform the context data from the call into an XDM for AEP.

In the input context some fields may be missing, so I did set up default values for those data elements that might be missing.

However in the generated XDM schema, the default values are not taken into account:
e.g. snippet from the modify data JSON:
"xdm": {
    "person": {
        "name": {
           "lastName": "{%%pii_lastname%%}",
           "firstName": "{%%pii_firstname%%}"
           }
         },... 

data element "pii_firstname" set up as a Context data field matching "firstname", default value set to "-"

Griffon log (no firstname field in contextdata):

"contextdata": {
        "country": "Belgium",
        "lastname": "Test",
        "language": "be_nl",
        ...

"xdm": {
        "homeAddress": {
          "country": "Belgium"
        },
        "preferredLanguage": "be-nl",
        "person": {
          "name": {
            "firstName": "",
            "lastName": "Test"
          }
        }

Anyone any idea if I did setup something wrong, or if I misinterpreted the "default" value processing?