The "$array" spec is not behaving as expected

[KilianLissSMRTR]

Here is some example code to reproduce the issue:

from json_converter.json_mapper import JsonMapper
from json_converter.json_mapper import json_object, json_array
from json_converter.post_process import prefix_with, concatenate_list
from json_converter.post_process import default_to
import json

exampleJson = {
    "STUDY_SET": {
        "STUDY": {
            "IDENTIFIERS": {
                "EXTERNAL_ID": [
                    {
                        "@namespace": "BioProject",
                        "@label": "primary",
                        "#text": "PRJNA269149"
                    },
                    {
                        "@namespace": "GEO",
                        "#text": "GSE63817"
                    }
                ]
            },
            "STUDY_LINKS": {
                "STUDY_LINK": {
                    "XREF_LINK": {
                        "DB": "pubmed",
                        "ID": "26495981"
                    }
                }
            }
        }
    }
}

specs = {
    '$on' : 'STUDY_SET.STUDY',
    'schema:identifier' : ['$array', [
        {
            '$on' : 'IDENTIFIERS.EXTERNAL_ID',
            '@type': ['', default_to, 'PropertyValue'],
            'schema:propertyID': ['', default_to, 'external_id'],
            'schema:value': ['#text'],
            'schema:valueReference' : ['@namespace']
        },
        {
            '@type': ['', default_to, 'PropertyValue'],
            'schema:propertyID': ['STUDY_LINKS.STUDY_LINK.XREF_LINK.DB'],
            'schema:value': ['STUDY_LINKS.STUDY_LINK.XREF_LINK.ID']
        },
    ], True],  # Keep this True for array mapping
}

transformedJSON = JsonMapper(exampleJson).map(specs)
print(json.dumps(transformedJSON, indent=4))

This is what I would expect it to output:

{
    "schema:identifier": [
        {
            "@type": "PropertyValue",
            "schema:propertyID": "external_id",
            "schema:value": "PRJNA269149",
            "schema:valueReference": "BioProject"
        },
        {
            "@type": "PropertyValue",
            "schema:propertyID": "external_id",
            "schema:value": "GSE63817",
            "schema:valueReference": "GEO"
        },
        {
            "@type": "PropertyValue",
            "schema:propertyID": "pubmed",
            "schema:value": "26495981"
        }
    ]
}

However it outputs with missing variables and values:

{
    "schema:identifier": [
        {
            "@type": "PropertyValue",
            "schema:propertyID": "external_id"
        },
        {
            "@type": "PropertyValue",
            "schema:propertyID": "pubmed",
            "schema:value": "26495981"
        }
    ]
}

I've tried a few alternative specs, but they either crash, or produce even more undesirable outputs.

Thanks in advanced for looking into this!