Database
[{"_id":"42.abc","ts_utc":ISODate("2019-05-27T23:43:16.963Z")},{"_id":"42.def","ts_utc":ISODate("2019-05-27T23:43:17.055Z")},{"_id":"69.abc","ts_utc":ISODate("2019-05-27T23:43:17.147Z")},{"_id":"69.def","ts_utc":ISODate("2019-05-27T23:44:02.427Z")}]
Query
db.collection.aggregate([{"$addFields":{"group":{"$arrayElemAt":[{"$split":["$_id","."]},0]}}},{$sort:{ts_utc:-1}},{"$group":{"_id":"$group","doc":{"$first":"$$ROOT"}}},{"$replaceRoot":{"newRoot":"$doc"}}])