Question

A trade-position service consumes a Kinesis stream keyed by `account_id`. Support reports a handful of accounts show a position that briefly went *negative* then corrected — an out-of-order apply of `debit`/`credit` events that are supposed to be strictly ordered per account. Dashboards: `IteratorAgeMilliseconds` is low; no lag. Recent context: ops increased the stream from 8 to 16 shards (a reshard / shard split) at 14:00 to handle growth, and the KCL consumer fleet scaled from 8 to 16 workers around the same time. The affected accounts are all ones whose `account_id` hash landed near a split boundary. Triage and explain the ordering violation.