What could cause a daily summary report generated by FortiSASE to contain very little data?

The reason the chosen answer is correct lies in the function of logging within FortiSASE. If the setting for logging allowed traffic is restricted solely to 'Security Events', it means that only security-related entries are captured in the logs. As a result, normal traffic activities that are not classified as security events would be omitted from the logs. This selective logging will lead to a daily summary report containing very little data since it doesn't account for all types of traffic processed by the system.

In contrast, if the FortiSASE system is in maintenance mode, it may not generate reports as expected, but it wouldn't specifically restrict the amount of data within a report already generated. Similarly, while unusually low network traffic might result in limited data generation, it’s not specific to the logging configuration that directly affects what is included in the summary report. System updates might temporarily impact reporting functionality, but again, they wouldn't affect the actual data captured and logged based on the defined policy settings. Thus, the key factor here is the restrictive logging setting that limits data collection to only security events.