$Reduce $Map Returning incorrect results and making other fields return undefined.

[gak10100]

import { aggregate } from "mingo";
import "mingo/init/system";

function main() {
  const result = aggregate(
    [{
      "skills":{
        "data": [
          {
            "name":"Respiratory Therapst",
            "categories":{
              "data":[
                {
                  "name": "Healthcare"
                }
              ]
            }
          },
          {
            "name":"Phlebotomy",
            "categories":{
              "data":[
                {
                  "name": "Skills (N-R) - DNU for Postings"
                },
                {
                  "name": "Healthcare"
                }
              ]
            }
          }
        ]
      },
      "location" : 'abcd'
  }],
  [
    {
      "$project" : {
        "specialties" : {
            "$reduce" : {
              "input" : "$skills.data",
              "initialValue" : [],
              "in" : {
                "$concatArrays" : [
                  "$$value",
                  {
                    "$map" : {
                      "input" : "$$this.categories.data",
                      "as" : "category",
                      "in" : {
                        "skill" : "$$this.name",
                        "category" : "$$category.name"
                      }
                    }
                  }
                ]
              }
            }
          },
          "location": "$location"
        }
      }
  ]);

  console.log(result[0]);
}

main();

Expected Result

{
    "specialties" : [
        {
            "skill" : "Respiratory Therapst",
            "category" : "Healthcare"
        },
        {
            "skill" : "Phlebotomy",
            "category" : "Skills (N-R) - DNU for Postings"
        },
        {
            "skill" : "Phlebotomy",
            "category" : "Healthcare"
        }
    ],
    "location" : "abcd"
}

Actual Result

{
  specialties: [
    { skill: 'Healthcare', category: 'Healthcare' },
    {
      skill: 'Skills (N-R) - DNU for Postings',
      category: 'Skills (N-R) - DNU for Postings'
    },
    { skill: 'Healthcare', category: 'Healthcare' }
  ]
}

When using $map inside a $reduce it is returning incorrect results and causing fields projected after the reduce map to be undefined.