Oracle AWR shows missing stats for SQL ID as top elapsed statement
I could think of eviction from shared pool, like your guess, or the statement running with statistics_level=basic. The statistics_level can be dynamically set on a per-session basis.


  Setting the STATISTICS_LEVEL parameter
  to BASIC disables the collection of
  many of the important statistics
  required by Oracle Database features
  and functionality, including:

*

  Automatic Workload Repository (AWR) Snapshots