You can go to the project where the task belonged to and scroll at the bottom of the task list. You should see "Complete tasks". Tap it and it should show you the task that were completed in that project.
I see the list of completed task. However, I do not see a search field. I have hundreds of completed tasks and it is not practical to scroll through the list to find the task I am looking for.
Unfortunately, that's not possible yet. We may add it in a future update though.
That is disappointing. Thanks for your quick response.