Out-of-order events post-reshard

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.