How do the fragments dynamically added in the later stage of mongodb cluster take effect on the original set of open fragments?

  mongodb, question

For example, my mongodb shard cluster originally had 2 shards. After opening the shards for the test.user set, insert the data into the test.user. Observation shows that the data has been evenly distributed on the original 2 shards. Then, a new shard has been added at the later stage. Then insert the data into the test.user again. It is found that only the original 2 shards still have data, but there is no data on the new shard!

If other sets such as test.role are opened after adding a slice, the inserted data can be evenly distributed over the 3 slices!

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