From poking at this for a few minutes, I can see /var/log/gaia_api_server.log contains interesting data:
17/06/22 15:50:30: MainThread: infra.urlGeneratorIS: DEBUG: Handle request: <class 'requests.sessions.LoginRequest'>
17/06/22 15:50:30: MainThread: infra.urlGeneratorIS: DEBUG: Execute validators
17/06/22 15:50:30: MainThread: infra.urlGeneratorIS: DEBUG: requestData = {'user': 'admin', 'password': '1qaz!QAZ'}
17/06/22 15:50:30: MainThread: validate: DEBUG: _validate json = {'user': 'admin', 'password': '1qaz!QAZ'}
17/06/22 15:50:30: MainThread: infra.urlGeneratorIS: DEBUG: All Active type: {'user': 'non_empty_ascii', 'password': 'non_empty_ascii'}
17/06/22 15:50:30: MainThread: infra.urlGeneratorIS: DEBUG: Verify permissions
17/06/22 15:50:30: MainThread: infra.pipeline: DEBUG: Execute commit function for class <class 'requests.sessions.LoginRequest'>
17/06/22 15:50:31: MainThread: infra.pipeline: DEBUG: Handling output
17/06/22 15:50:31: MainThread: objects.sessions: INFO: Remote authentication succeed for user:admin
17/06/22 15:50:31: MainThread: app: INFO: Request for endpoint /v1.6/login [method: POST], for source 10.0.3.18, SUCCEEDED [duration 329ms]
17/06/22 15:50:46: MainThread: infra.urlGeneratorIS: DEBUG: Handle request: <class 'requests.sessions.LogoutRequest'>
17/06/22 15:50:46: MainThread: infra.urlGeneratorIS: DEBUG: Execute validators
17/06/22 15:50:46: MainThread: infra.urlGeneratorIS: DEBUG: requestData = {}
17/06/22 15:50:46: MainThread: validate: DEBUG: _validate json = {}
17/06/22 15:50:46: MainThread: infra.urlGeneratorIS: DEBUG: All Active type: {}
17/06/22 15:50:46: MainThread: infra.urlGeneratorIS: DEBUG: Verify permissions
17/06/22 15:50:46: MainThread: infra.pipeline: DEBUG: Execute commit function for class <class 'requests.sessions.LogoutRequest'>
17/06/22 15:50:46: MainThread: infra.pipeline: DEBUG: Instantiate response class
17/06/22 15:50:46: MainThread: app: INFO: Request for endpoint /v1.6/logout [method: POST], for source 10.0.3.18, SUCCEEDED [duration 10ms]
I added a newline between the login and logout. Logins record when the user authentication works, but no unique session identifier. Logouts don't record who logged out or any kind of session identifier. Passwords are recorded in cleartext (twice!), which is ... not great.
/var/log/audit/audit.log shows logins, but no session identifier and no logouts.