If by session id you’re referring to domain_sessionid, then this is the correct behaviour - that field is specific to web and shouldn’t be populated for mobile events.
Instead, setting sessionContext: true for the react-native tracker will attach the client session context to events, which contains the session ID for mobile.
You will find this in the com_snowplowanalytics_client_session_1 field/column in Kibana/Snowflake, the atomic.com_snowplowanalytics_client_session_1 table in Redshift, or the com_snowplowanalytics_client_session_1_0_1 column in BigQuery.