Great question. Updating an additional Task created in a Task Template to entities the Template is already linked to is possible, but it’s just a little hidden. (Whew! That was a mouthful.)
NOTE: the following functionality has changed after the 8.17.0.5743 release. Please see the solved response for details.
Once you’ve added the new Task to the Task Template, just head to your Task Template page > right-click on the Task Template you added the Task to > and then select “Update Entities with New Tasks”
Also note that this updates are only additive. So let’s say you delete a Task in a Task Template already linked to entities, and run the same update. In this case, the deleted Task will not be deleted from those entities. Only additions will be applied.
Be sure to check out some more information on this, along with other Task Template functionality in our Admin Guide here.