With the current implementation of the Xamarin.Forms DataForm, this functionality is not supported. I have logged your suggestion as a feature request in our Ideas & Feedback portal - DataForm: Present a mechanism to Collapse/Expand groups. I suggest you vote for the item and also follow it in order to receive automatic notifications when some major updates are available. I have also added 500 points to your account for bringing up the functionality.
In the meantime, it is possible to achieve the behavior for the Android platform by creating a custom renderer. I have attached a sample for your reference. Please have a look at it and consider such step at your end as well.
Have a great week, Clayton.
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