Hi Yongpyo,
If you mean a microflow triggered by a scheduled event? That has no account that triggers it as there is no user action to trigger this event, but a time trigger.
If you debug a scheduled event you will also see there is no currentSession or currentUser. Hence also the system owner will be empty on creation of an object, or the object of a retrieval associated to the current user.
You can also not use any front end actions (they will not have use) in scheduled events, like show a page or show a message.