In FirebaseAnalytics data exported to BigQuery, there are some events that have null ga_session_id

I'm using FirebaseAnalytics data exported to BigQuery to analyze our app.
To check user transitions, I write sql like below.

with users_with_screen_view_and_session_id as (
  SELECT
    user_id,
    user_pseudo_id,
    event_date,
    event_timestamp,
    event_name,
    platform,
    case
      when event.key = 'firebase_screen'
        and event.value.string_value is not null then event.value.string_value
      when event.key = 'firebase_screen'
        and event.value.int_value is not null then cast(event.value.int_value as string)
      else null
    end as firebase_screen,
    case
      when event.key = 'ga_session_id'
        and event.value.string_value is not null then event.value.string_value
      when event.key = 'ga_session_id'
        and event.value.int_value is not null then cast(event.value.int_value as string)
      else null
    end as session_id
  FROM
    `{my_project}.analytics_{my_id}.events_*`  as main
    ,unnest(event_params) as event
    WHERE _TABLE_SUFFIX BETWEEN '20191001' AND '20191001'
    and event_name = 'screen_view'
),
users_with_screen_view_agg_firebase_screen as (
  select
    user_id,
    user_pseudo_id,
    event_date,
    event_name,
    platform,
    event_timestamp,
    max(firebase_screen) as firebase_screen,
    max(session_id) as session_id
  from
    users_with_screen_view_and_session_id
  group by
    user_id,
    user_pseudo_id,
    event_date,
    event_name,
    platform,
    event_timestamp
)
select
  *
from
  users_with_screen_view_agg_firebase_screen
;

I thought every event have ga_session_id, but in this query result there are some rows that's ga_session_id is null.

In what case does ga_session_id value become null?

The image of query result is here. enter image description here