There are two common reasons a task might seem to vanish:
It wasn't Committed: In Tasks, you must press Enter after typing to save a task, or edits to the task. If you navigate away before hitting Enter, the task won't save.
- It was Deferred: If you accidentally clicked Defer, the task is hidden until a future date. Check the Deferred view in the sidebar to see if your task is waiting there to resurface later.