iOS: ‘Count’ not available as a Summary Calculation?

I have a calculation field in one of my forms that returns a timestamp (which I format to show the time only). I want to set the Summary Calculation (under Field Options on iOS) for this field to Count. On the Mac version of Tap Forms, I can do this and it behaves as expected. But on the iOS version, it’s not possible to set this to Count. If I set the Summary Calculation to Count on my Mac, on iOS it doesn’t display anything where the summary calculation should be (i.e. at the bottom of a table column in a Link to Form field). The attachments show a screenshot from the same record on the same form in both Mac and iOS. I want Count to show for the Time column.