You can find an example of the issue in this jsFiddle https://jsfiddle.net/ewaldhofman/hjkxcr45/1/
In the example, I have a simple datasource which gets local data, but uses ServerAggregates to show the issue that I am running into. The issue is that I can't figure out how to update the grandtotal (footerTemplate) when using pushUpdate.
I use pushUpdate to improve the performance of updating the grid, and everything works (including group totals - which I have enabled in my environment, although not available in the sample) except for the grand total.
I have tried to call the grid.refresh() and the dataSource.read(), but nothing seems to work.