I've found out the reason behind difference in behavior. RadContextMenu is a child of RadGridView while ToolTip is in a different branch build around PopUp primite so we access columns by Ancestor source. I've write a work-around based on this code:

https://stackoverflow.com/a/1759923

public class HiddenColumnsToTooltipConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value != null && value is Camera camera)
        {
            try
            {
                var window = Application.Current.Windows.OfType<Window>().SingleOrDefault(x => x.IsActive);
                var columns = ((RadGridView)FindChild(window, $"{parameter}", typeof(RadGridView))).Columns;

                ...

                return result;
            }
            catch
            {

            }
        }

        return "Nothing to show";
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return Binding.DoNothing;
    }

    public static DependencyObject FindChild(DependencyObject reference, string childName, Type childType)
    {
        DependencyObject foundChild = null;
        if (reference != null)
        {
            int childrenCount = VisualTreeHelper.GetChildrenCount(reference);
            for (int i = 0; i < childrenCount; i++)
            {
                var child = VisualTreeHelper.GetChild(reference, i);

                if (child.GetType() != childType)
                {
                    foundChild = FindChild(child, childName, childType);
                    if (foundChild != null) break;
                }
                else if (!string.IsNullOrEmpty(childName))
                {
                    var frameworkElement = child as FrameworkElement;

                    if (frameworkElement != null && frameworkElement.Name == childName)
                    {

                        foundChild = child;
                        break;
                    }
                }
                else
                {
                    foundChild = child;
                    break;
                }
            }
        }
        return foundChild;
    }