The doc:
https://docs.python.org/dev/library/asyncio-task.html#asyncio.gather
"Cancellation: if the outer Future is cancelled, all children (that have not completed yet) are also cancelled. If any child is cancelled, this is treated as if it raised CancelledError – the outer Future is not cancelled in this case. (This is to prevent the cancellation of one child to cause other children to be cancelled.)"

(Sorry, I didn't read your issue yet.)