We tried to delegate task to user which is not valid LDAP user (not exists in our LDAP) and API call didn't throw exception.

We have looked the source code for DelegateTaskCommand

and found that we are not checking method return from doCallbackUserOperation(targetEntityId, context);
method. (targetEntityId check is ignored)

Result:
The human task state is moved to next state (Reserved), but ActualOwner task property is empty.