Apologies to all of you.
Tasks should NOT disappear!
We are having trouble reproducing this issue - so if anyone has a reproducible case please share.
From what we can tell the task item, that is stored in the project (aprx file), no longer exists.
So you end up with a project that reference a task item that no longer exists (hence the broken source).
Until we fix it please save a backup: save the task item to a task file.
Amir
Jason Camerano - FYI