Enteries from pr_sys_lock table are only deleted when the lock is released. pxExpireDateTime is not an indication of lock released rather it converts the lock to a soft lock.
pxExpireDateTime is evaluted based on default timeout provided in Case locking settings. After this time out is passed, pxExpireDateTime will reach and lock will be converted to soft lock. Soft lock means, that any other user can now acquire the lock and submit the work and then lock will be released. Or if the same user comes back who initially had the lock and submits, it will then submit the assignment and release the lock.