After deleting the users from the UI they are only soft-deleted. If you look at the user id and find the matching element in your elements table, you will see that this element has a populated dateDeleted.
Run garbage collection (./craft gc) to clean up and completely remove everything.
You could also use your general config file to set the softDeleteDuration to a very low value, so that soft deleted elements get garbagecollected quickly:
'softDeleteDuration' => 300, // 5 Mins
or if you're up to the command line, use
./craft gc --delete-all-trashed
to immediately hard delete all soft deleted items
This happens when you go from old craft versions, in the latest versions you have to have the config/db.php file like this:
'dsn' => App::env('DB_DSN') ?: null,
'driver' => App::env('DB_DRIVER'),
'server' => App::env('DB_SERVER'),
'port' => App::env('DB_PORT'),
'database' => App::...