A while (several days usually) after starting up the database (sharded cluster), newly inserted time series collection will have exactly (as far as I personally observed) as many buckets as documents, regardless of granularity or metafield settings. The stats() command shows that the buckets were closed due to cache pressure (numBucketsClosedDueToCachePressure).

I also experience the issue with after deleting a time series collection, and inserting either the same or other collections.

This behavior has also been documented/observed in:
https://www.mongodb.com/community/forums/t/suboptimal-bucket-creation-in-timeseries-collection-due-to-cache-pressure/249863
https://dba.stackexchange.com/questions/336013/mongodb-timeseries-data-at-high-load-causes-cache-pressure-in-time
https://www.mongodb.com/community/forums/t/why-timeseries-bucket-count-is-different-on-2-independent-environment-with-same-data/265066/