The “tran” namespace has evicted records in the past. Eviction is basically early expiration when HWM configurations are breached (see the link for details). I believe this would explain the behavior you are seeing.
You can configure some a set to not be evictable, details can be found here: https://www.aerospike.com/docs/operations/configure/namespace/retention/index.html#set-disable-eviction.