The data is stored as:

{    "_id":\{"$oid":"5e7b22bd2a7912a5a3b73d79"}

,
   "manufacturerID":"19193",
   "units":[
                  {
                      "assetsReserved":{
                                "departmentCode" : "PHY_DEPTS",
                                "assets":{"primaryID":"1234"}}
                  },
                 {
                          "departmentCode" : "PHY_DEPTS",
                          "assetsReserved":{"assets":{"primaryID":"4567"}}
                 }
             ]
}
The requirement is to get all the units where primaryID matches the "1234"

The mongo.Pipeline is made up of 
 
matchStage := bson.D{primitive.E{Key: "$match", Value: bson.D{primitive.E{Key: "manufacturerID", Value: "19193"}}}}

projectStage := bson.D{
                                      {"$project", bson.D{
                                                              {"units", bson.D{
                                                                           {"$filter", bson.D

{                                                                            \{"input", "$units"}

,
                                                                           {"as", "units"},
                                                                           {"cond", bson.D{
{"$or", bson.A{                                                                                                        bson.D{{"$eq", bson.A

{"$$units.assetsReserved.departmentCode", "1234"}

}},
bson.D{{"$eq", bson.A

{"$$units.assetsReserved.assets.primaryTrackingID", "1234"}

}},
}},
}},
}},
}},
}},
}
unwindStage := bson.D

{ \{"$unwind", "$units"}

,
}

The search based on 

units.assetsReserved.departmentCode works

If we pass "PHY_DEPTS" we get the response, however 

units.assetsReserved.assets.primaryTrackingID doesn't return any response

If we pass "1234"
 
"$elemMatch" works fine with assetsReserved.assets.primaryTrackingID and is able to return result however it returns only 1 record even when multiple records with same primaryTrackingID exists which is the expected behavior based on documentation.
Looks like issues with $filter and cond that its not able to filter when subdocument contains an array