workaround is to override the OnInitialized of the custom column:
protected override void OnInitialized(EventArgs e)
{/* //if XAML assemblies
Theme theme = StyleManager.GetTheme(this);
Type themeType = null;
if (theme != null)
{
themeType = theme.GetType();
}
this.DefaultStyleKey = new ThemeResourceKey() { ElementType = typeof(Telerik.Windows.Controls.GridViewColumn), ThemeType = themeType };*/
}
The bug can also occur on Custom Controls not derived of any telerik class. But it is also fixed by this workaround.