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.
As said in this thread https://github.com/telerik/UI-For-UWP/issues/332 the decimal-separator is bugged when used in windows 10 1809, at least if the decimal-separator system-setting is other then a dot, e.g. in Germany which uses comma.
The Bug seems to be solved in the referred Branch by Nasko https://github.com/telerik/UI-For-UWP/tree/Nasko/update-sdk yet it is not available in NuGet, not even as a prerelease.
Also setting the decimal-separator manually is impossible, yet in 2016 it was said this change will be implemented https://www.telerik.com/forums/problem-with-decimal-separator
However, is there a schedule, when the Bugfix will arrive in NuGet? The recent version is 184.108.40.206 from October 17th 2018 while the fixed branch is from November 2nd 2018.