I have retested this with a default grid on the form and the code:
procedure TForm1.FormCreate(Sender: TObject);
begin
TMSFMXGrid1.RandomFill(false,10);
TMSFMXGrid1.AutoNumberCol(0);
TMSFMXGrid1.Options.Grouping.MergeHeader := true;
TMSFMXGrid1.Group(1);
end;
and I can only see cell text of cells that get a node is now displayed which is by design as the cell has a node. The node remains visible regardless of the group header merging.
Because is very difficult explain in text the problem, I've create a video to show it.
https://youtu.be/MuN5xdlLx0s
As you see, the problem is when you use some Right Fixed Columns and the total width of the rest of columns is greater than the space from the left to the position of the first Right Fixed Column.