I just tried and see the same thing, looks like a bug.
Internally “node paths” are used to control what’s visible and what’s filtered out. When you focus on an item FoldingText generates an appropriate node path to select that item and it’s contents.
Unfortunately in this case it’s generating an incorrect node path. It’s complicated but “Task” is also a keyword that means find all items of type “Task” and the method for generating the node path isn’t taking that into consideration.
I think now you’ll just have to use one of your above workarounds.
Also if you have time you might take a look at the in progress “FoldingText for Atom” project that I’m working on. I think it’s especially well suited to making task lists, and it’s also much clearer in the way that it implements focusing. (This particular type of bug would be impossible).