From the linked ticket:

All newly created time-series collections have timeseriesBucketsMayHaveMixedSchemaData set to false. This flag never gets changed as time-series inserts through the view can never result in mixed-schema buckets. However, one can still insert mixed-schema buckets through the system.buckets collection (direct write) and we wouldn't update the flag to true. mongodump/mongorestore does this. We should check whether buckets inserted via direct writes contain mixed-schema data and set the flag appropriately.