With DEBUG

└── 09:19:54 | INFO    | Task 'generate_data': Starting task run...
└── 09:19:54 | DEBUG   | Task 'generate_data': Handling state change from Pending to Mapped
└── 09:19:54 | INFO    | Task 'generate_data': Finished task run for task with final state: 'Mapped'
└── 09:19:54 | INFO    | Task 'generate_data[0]': Starting task run...
└── 09:19:54 | DEBUG   | Task 'generate_data[0]': Handling state change from Pending to Running
└── 09:19:54 | DEBUG   | Task 'generate_data[0]': Calling task.run() method...
└── 09:19:55 | DEBUG   | Task 'generate_data[0]': Handling state change from Running to Success
└── 09:19:55 | INFO    | Task 'generate_data[0]': Finished task run for task with final state: 'Success'
└── 09:19:55 | INFO    | Task 'write_data': Starting task run...
└── 09:19:55 | DEBUG   | Task 'write_data': Handling state change from Pending to Running
└── 09:19:55 | DEBUG   | Task 'write_data': Calling task.run() method...
└── 09:19:55 | DEBUG   | Task 'write_data': Handling state change from Running to Success
└── 09:19:55 | INFO    | Task 'write_data': Finished task run for task with final state: 'Success'
└── 09:19:55 | INFO    | Task 'generate_other_stuff': Starting task run...
└── 09:19:55 | DEBUG   | Task 'generate_other_stuff': Handling state change from Pending to Failed
└── 09:19:55 | INFO    | Task 'generate_other_stuff': Finished task run for task with final state: 'Failed'

Injecting a debugger into the state transition:

ipdb> task
<Task: generate_other_stuff>
ipdb> current_state
<Pending>
ipdb> next_state
<Failed: "At least one upstream state has an unmappable result.">

Seems like you need to add

unmapped(…)

around the upstream task in

upstream_tasks

I suppose it makes sense, but the lack off error message was a bit confusing. With a state handler on the task, I could see the Failed message, not sure why it doesn’t show up in the logs that way