[{"_id":ObjectId("5e9d5785e4c8343bb2b455cc"),"name":"Jenny Adams","report":[{"category":"Beauty","status":"submitted","submitted_on":[{"_id":"xyz","timestamp":"2022-02-23T06:10:05.832+00:00"},{"_id":"abc","timestamp":"2021-03-23T06:10:05.832+00:00"}]},{"category":"Kitchen","status":"submitted","submitted_on":[{"_id":"mnp","timestamp":"2022-05-08T06:10:06.432+00:00"}]}]},{"_id":ObjectId("5e9d5785e4c8343bb2b455db"),"name":"Mathew Smith","report":[{"category":"Household","status":"submitted","submitted_on":[{"_id":"123","timestamp":"2022-02-23T06:10:05.832+00:00"},{"_id":"345","timestamp":"2021-03-23T06:10:05.832+00:00"}]},{"category":"Garden","status":"submitted","submitted_on":[{"_id":"567","timestamp":"2022-05-08T06:10:06.432+00:00"}]},{"category":"BakingNeeds","status":"submitted","submitted_on":[{"_id":"891","timestamp":"2022-05-08T06:10:06.432+00:00"}]}]}]

db.collection.aggregate([{$unwind:"$report"},{"$addFields":{"timestamp":{$arrayElemAt:["$report.submitted_on",-1]}}},])