Query-based bucket reopening relies on a {<metaField>: 1, <timeField>:1} index to execute efficiently. If the index does not exist, query-based reopening will not be used. This index is not created when a time-series collection is created without a meta field.

Speaking with dan.larkin-york@mongodb.com, we think we could do something performant in this case using the clustered index. This ticket is to investigate this.