I created a post in the forums but I don't know how much attention that particular forum gets.
I have created a simplified project on GitHub that reproduces the problem.
I'm not sure if the problem is with the control or the framework, but my example project clearly demonstrates the problem.
I suspect the problem is related to binding the Maximum property of the RadNumericBox. Items in the "Two" group have a higher maximum than items in the "One" and "Three" groups. While scrolling through the list, some items in the "Two" group get their values changed to the maximum from the other groups. It's also odd that the values that change are changed twice. The Debug Output window shows when and how each value is changed while scrolling through the list.
If any of this is not clear, I am happy to provide more detail, however, I'm sure the example project will make the problem clear.