> db.createCollection("ts", { timeseries: { timeField: "time", metaField: "meta", granularity: "seconds" }})
> db.ts.getIndexes()
[{"v" : 2, "key" : {"meta" : 1,"time" : 1},"name" : "meta_1_time_1"}]
> db.system.buckets.ts.getIndexes()
[{"v" : 2,"key" : {"meta" : 1,"control.min.time" : 1,"control.max.time" : 1},"name" : "meta_1_time_1"}]
> // insert some data into "ts" so it's not empty
> db.ts.explain().aggregate([{$sort: {meta: 1, time: 1}}, {$group: {_id: "$meta", last: {$last: "$val"}}}])

The plan won't have DISTINCT_SCAN because the default index {meta:1, time: 1} doesn't match the query sorting requirements, but it will have a bucket-level $group with $last accumulator, followed by unpack, followed by event-level $group.

A pipeline like this should be fully lowerable to SBE but I'm not seeing the bucket-level $group being lowered in my tests. We should investigate this and either keep the whole query in the classic engine or ensure that it gets lowered beyond the unpacking stage. This is likely related to SERVER-79066.