Why won't Oracle's Unified Audit Trail pick up the ALTER SESSION statement?

Question

I have created a simple policy in Oracle 19c to audit when a user uses the "ALTER SESSION" privilege:

CREATE AUDIT POLICY ALT_SES_PLCY
  Privileges
    ALTER SESSION;

AUDIT POLICY ALT_SES_PLCY;

After creating and enabling the policy, I disconnect from the database, then reconnect. I then execute the following line:

ALTER SESSION SET CURRENT_SCHEMA = TEST_USER;

Then I check the audit trail:

SELECT * FROM UNIFIED_AUDIT_TRAIL
ORDER BY EVENT_TIMESTAMP_UTC DESC;

Yet there exist no record of the ALTER SESSION privilege being used. What gives? Why is this not being picked up?