Buggy buckets when using custom bucketing with timeseries

Testing bucketing on a timeseries collection. Why are multiple buckets being created even though I am inserting exactly the same data?

Collection Info:
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Data that I am inserting:
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Multiple buckets for the same timestamp and metadata!

Inserted the same set of recs 2x. Instead of 2 buckets, 3 buckets are getting created. A list of 2 records - inserted 2x - is creating 3 buckets? How?