[{"_id":"1","shipment_events":[{"_id":"2","event_at":"2022-04-09 15:00:00 UTC"},{"_id":"3","event_at":"2022-04-09 18:00:00 UTC"}]},{"_id":"2","shipment_events":[{"_id":"2","event_at":"2022-04-09 15:00:00 UTC"},{"_id":"3","event_at":"2022-04-09 18:00:00 UTC"}]}]

db.collection.aggregate([{$unwind:"$shipment_events"},{$sort:{_id:1,"shipment_events.event_at":-1}},{$group:{_id:"$_id",event_id:{$first:"$$ROOT.shipment_events._id"},event_at:{$first:"$$ROOT.shipment_events.event_at"}}}])