Definition says: These states enable coordination of task status with off-the-shelf workflow solutions that support automation of tasks. The values are constrained to ready | cancelled | completed because the task come to the patient in the ready status and the patient can cancel it or complete it.

So this seems to imply that it would not be OK for a Patient app to update Task without changing the status and that the only status changes allowed are Ready to Cancelled or Ready to Complete.  Perhaps invariants would help here to make this more clear and a state machine.