Thanks for the sample, we'll try to fix this problem to the end of this week, to the nesrest FlyGrid.Net update.
UPD: The problem is in using FlyGrid.Columns.LeftFixedColumns > 0 when some of the fixed columns has non-Exact ColumnFitMode. Currently you can turn TreeViewcolumn.FitMode to Exact or set FlyGrid.Columns.LeftFixedColumns to 0 to solve this problem.