This is a feature request and a question, in case someone knows a secret I do not.
It would be very useful to collapse groupings in perspectives on my iPhone the same way I do on my iPad and Mac. The main reason, in my case, is to be able to filter my “Scheduled” custom perspective to only show things with a future defer date. At present, I have to scroll through a great deal of unavailable tasks with a past defer date. (This is similar to the built-in Forecast perspective, though that for me is unusable since I cannot exclude items and are forced to show due items as well. Although, the ability to uncheck “show due” items in Forecast, along with the ability to exclude certain Contexts, would be an alternative solution.)