The kworker processes are not related to Unbound.
They are Linux kernel worker threads that handle various asynchronous tasks in the background.
I am actually glad to see that I’m not the only one experiencing issues with leases.db.
I tried to explain this here, but at the time it seemed that I was the only person affected:
From my observations, the first IP address recorded for a hostname persists in the database, even if that hostname later receives a different IP address.
The entry is only removed when the DHCP lease is released or expires. However, this does not always happen, so the stale record can remain in leases.db indefinitely unless it is removed manually.