When Querying just for tasks that are marked for today in python: 
client.tasks.find_all({ 'assignee_status':'upcoming','workspace': 000000000,'assignee':'me' ,'completed_since':'now'}, page_size=100)
I get a response of all tasks the same as if I would not have included assignee_status
client.tasks.find_all({'workspace': 000000000,'assignee':'me' ,'completed_since':'now'}, page_size=100)
The workspace space has around 5 task that are marked for today.
Thank you,
Greg
You actually can't filter by assignee_status at all - if you pass the parameter it is silently ignored. We could change it so that unrecognized parameters result in errors, which would help make this clearer.
Asana API querying by assignee_status