To reproduce: 
1. Add a RadSplitContainer with two split panels.
2. Add a RadCollapsiblePanel to the left split panel and set the RadCollapsiblePanel.Dock property to Fill.
3. Add a RadGridView inside the RadCollapsiblePanel  and set the RadGridView.Dock property to Fill.
4. Populate the grid with data.
5. Run the application and move the splitter to increase the RadCollapsiblePanel's size. The RadGridView is resized correctly.
6. Move the splitter to shrink the RadCollapsiblePanel. As a result the grid is not resized correctly. 

Workaround:

Subscribe to the RadSplitContainer.SplitterMoved event and use the  following code:

this.RadCollapsiblePanel1.EnableAnimation = false;
this.RadCollapsiblePanel1.IsExpanded = !this.RadCollapsiblePanel1.IsExpanded;
this.RadCollapsiblePanel1.IsExpanded = !this.RadCollapsiblePanel1.IsExpanded;
this.RadCollapsiblePanel1.EnableAnimation = true;