[{"objectId":"aaaaa","userId":"userId1","processedAt":"2021-06-01T12:16:49.349Z"},{"objectId":"bbbbb","userId":"userId1","processedAt":"2021-10-11T12:16:49.349Z"},{"objectId":"ccccc","userId":"userId1","processedAt":"2021-10-25T12:16:49.349Z"},{"objectId":"ddddd","userId":"userId1","processedAt":"2021-09-23T12:16:49.349Z"},{"objectId":"eeeee","userId":"userId1","processedAt":"2021-09-12T12:16:49.349Z"}]

db.collection.aggregate([{"$addFields":{"processedAt":{"$toDate":"$processedAt"}}},{"$setWindowFields":{"partitionBy":{"$dateToString":{"date":"$processedAt","format":"%Y-%m"}},"sortBy":{"processedAt":-1},"output":{"rank":{$rank:{}}}}},{$match:{rank:1}},{"$project":{rank:false}}])