Still having same issue here.
Most tests work fine, except when "not" is introduced, it seems.

im facing the same problem.

test.expect(not xo('xxxoo'), 'False expected')
             not    False             ^ 
                True            This is only shown if xo('xxxoo') returned True and not xo('xxxoo') was False instead

So far, you understood how it works. The error message is shown if the first argument of test.expect isn't True.