The structure like

workflow = chain(
    t.task1.s(),
    chord(
        [
            t.task2.s(),
            chain(
                t.task3.s(),
                chord(
                    [t.task4.s(), t.task5.s()],
                    t.task6.s()
                )
            )
        ],
        t.task7.s()
    )
)

never succeeds. Task t.task7 is never called, while other tasks succeed just fine.

Replacing chords with groups did nothing. When I remove the inner chord, workflow finishes without any problems. Workflow also succeeds if I remove an inner chain.

The exact code that can be used to reproduce this issue as well as log messages are in this gist