Able to query deleted records from recycle bin but not to undelete them

As a SystemAdmin with no role assigned in production,

Deleted parent account (which caused related event or task records deleted itself): I've run a batch, which do dml and send other sobject records to be dml in other batch from finish. I had to update the event and task records to assign a different parent account but the batch(failed) and finish continued to run another batch to delete their parent account(succeeded).

I used the Dynamic dml [Database.update(activitiesToUpdate, true) and Database.delete(accountsToDelete, true)]

I'm able to query those task and event records deleted implicitly with following soql

List<Task> tsks = [select id from task where isDeleted=true all rows];

system.assert(tsks.size()>0); //success

but unable to undelete them

undelete tsks; //throws an error that Those task record doesn't exist in recycle bin

FYI: i don't want to undelete parent account records, if it is required to undelete them can we remove the parent account id from record and then undelete, is this possible?

@* Salesforce Developers *