In the current CollectionView implementation, the CollectionView will start with all the groups expanded. The only way to have any form of preference is to programmatically interact with the DataView after-the-fact https://docs.telerik.com/devtools/maui/controls/collectionview/grouping/expand-collapse

Requested Feature
A better approach that I am requesting a feature for is to have a property available for the CollectionView that sets this value ahead of time.

For example, you could add it as a BindableProperty on the GroupDescriptor class and on the GroupDefinition is to have an IsExapanded property.