Based on the knowledge sources, if you've set your Mixpanel project to use property-based sessions and the session ID property is missing from an event, that event will not be used in calculating Session Start and End events.
According to the documentation on Property-Based Sessions:
Events that do not contain a "Session Id" value are not used in calculating Session Start and End events.
This means that any events without the specified session ID property will essentially be ignored when Mixpanel is determining session boundaries. Only events that contain the session ID property will be considered part of a session and contribute to the Session Start and Session End virtual events.
It's also worth noting that events with an empty session ID value (e.g., "session_id": "") will be tracked and session events will be created upon transition to a different session ID value.
Sessions: Compute sessions based on existing event data
