moodle config.php session_handler_class was set to '\core\session\memcached' in prior version. and auth_ldap\task\sync_task was working as expected.

As memcache is no longer supported.  switched to use redis:

$CFG->session_handler_class = '\core\session\redis';

it worked when set and saw redis was populating.   However, when Users were to-be-deleted:

Execute scheduled task: LDAP users sync job (auth_ldap\task\sync_task)
... started 09:17:25. Current memory use 2.8 MB.
Debugging increased temporarily due to faildelay of 86400
Connecting to LDAP server...
Creating temporary table tmp_extuser
...............Got 1690 records from LDAP
User entries to be removed: 7
... used 1761 dbqueries
... used 2.4485740661621 seconds
Scheduled task failed: LDAP users sync job (auth_ldap\task\sync_task),Setting up of redis session failed. Please notify the server administrator.
Backtrace:

• line 759 of /lib/classes/session/redis.php: call to core\session\redis->init()
• line 625 of /lib/classes/session/redis.php: call to core\session\redis->init_redis_if_required()
• line 576 of /lib/classes/session/manager.php: call to core\session\redis->get_sessions_by_userid()
• line 1005 of /lib/classes/session/manager.php: call to core\session\manager::get_sessions_by_userid()
• line 3675 of /lib/moodlelib.php: call to core\session\manager::destroy_user_sessions()
• line 812 of /auth/ldap/auth.php: call to delete_user()
• line 55 of /auth/ldap/classes/task/sync_task.php: call to auth_plugin_ldap->sync_users_update_callback()
• line 410 of /lib/classes/cron.php: call to auth_ldap\task\sync_task->execute()
• line 194 of /admin/cli/scheduled_task.php: call to core\cron::run_inner_scheduled_task()

Potential coding error - existing temptables found when disposing database. Must be dropped!

I then saw a flood of <prefix>[0-9]* table entries appear in the database.  Moodle Database is Oracle 19c.