I just tested this and was unable to replicate your scenario. I have a parent task with a context of "Home" and a subtask of this with a context of "Phone". If I go to the context view and click on the "Phone" tab, my subtask is in there.
Keep in mind that I have my filters set to "flatten subtasks". If you are using "nest subtasks" then the subtask will be hidden inside the parent as intended.