I doubt if it’s a “bug” in Asana’s eyes. I suspect that once this behavior is completely understood, they will say it’s “working as designed”. I’m guessing what has happened for you, is that you created a task and made it public, by “homing” it in a public project. Someone else had access and then multi-homed it to a private project, THEN took the fatal step of de-homing the task from the original, public project, leaving it only in the private project. Voilà - a task you created, that you know still exists, but which you can no longer access.
In Asana, it is 100% possible for anyone who is able to edit a task, to also delete that task, or make changes tantamount to deletion. If this is what has happened, and you can’t guess who may have done it, you can have recourse to your Asana admin or to Asana support. Prevent Users from Deleting Tasks