On https://www.mongodb.com/docs/manual/reference/audit-message/ the 'uuid' field of audit log messages is described as the following:

Document that contains a universally unique identifier (UUID) for the audit message. The $type field specifies the BSON subtype for the $binary field. The $type value of 04 specifies that the $binary field contains a UUID.

This gives the impression that a given UUID will only appear once in any file.  This is incorrect.  The field identifies a UUID associated with the client connection which generated the audit event.  This value will be repeated across multiple records in a given audit file, once for each event generated by a single client connection.  Grouping events by common UUIDs allows the actions of a single client to be associated together into a single user session.