This error occurs for both viewful and viewless timeseries collections. The slow query log for an aggregation on an untracked timeseries collection on a sharded cluster will have the collectionType be "normal" and not "timeseries". This is because mongos sends the request to the shards, which will execute the full query and mongos never knows that the collection was a timeseries collection. Repro is in the comments.

This ticket should also add tests for slow query logs on mongos in general.