To reproduce:
splitPanel1.SizeInfo.SizeMode = Telerik.WinControls.UI.Docking.SplitPanelSizeMode.Absolute;
splitPanel1.SizeInfo.AbsoluteSize = new Size(300, this.radSplitContainer1.Height);
splitPanel1.SizeInfo.MinimumSize = new Size(80, 0);

radSplitContainer1.UseSplitterButtons = true;
radSplitContainer1.EnableCollapsing = true;

- Start the application and collapse the panel with the button.
- The panel is collapsed, but cannot be expanded to its previous position.