When initial grouping is set through the state, the GroupFooterTemplate does not render. One has to perform some action for refreshing the Grid in order to display the template - for example, collapsing/expanding a group.
For reference, the GroupHeaderTemplate renders correctly.
Reproduction: https://blazorrepl.telerik.com/wRaJFgFy10vagYnf05
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ADMIN EDIT
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The GroupFooterTemplate renders with a delay after the Grid. A possible workaround for the time being is to refresh the Grid after it has been initialized to ensure the GroupFooterTemplate will be displayed.