Indeed, it must be a reset notification sent from the app (there's a third party API that I use), because the datasource is read again after the action I'm doing.

So my only option is now to get all the groups that are expanded/collapsed, and restore the state of the app.

My next question is : how can I know which rows are expanded or collapsed ?
I check the following fields, but they do not seem to be the good ones because they are set to "false" even when the group is expanded (I checked at debug time) :
in UltraGridrow : Expanded, HasExpansionIndicator, IsExpanded, and also ShowAsExpanded in a sub-hierarchy...

For those interested here's what I did :

Before calling the function causing the reset, I store the state of all the GroupByRows in a Dictionnary (called GroupByRowsState, storing the Value of the GroupByRow, and the Expanded state).
I also add a callback on the "InitializeGroupByRow" event.

In the callback I only set the Expanded state if it exists, so as not to interfere too much :
if (GroupByRowsState.ContainsKey((string)e.Row.Value))
{
  e.Row.Expanded = GroupByRowsState[(string)e.Row.Value];
  GroupByRowsState.Remove((string)e.Row.Value);
}

And it seems to work perfectly as of now.

re-rechecked, and yes the Expanded field in the UltraGridGroupByRow is correct...
So that's what I'll do : store the state of all GroupByRows, and then restore it after the screen refresh.

Thanks for your insight.