Workaround for this bug is setting Style.Triggers for this scenario.

<Style BasedOn="{StaticResource RadRibbonSplitButtonStyle}" TargetType="telerik:RadRibbonSplitButton">
  <Style.Triggers>
    <MultiTrigger>
      <MultiTrigger.Conditions>
        <Condition Property="IsChecked" Value="True"/>
        <Condition Property="IsEnabled" Value="True"/>
        <Condition Property="CurrentSize" Value="Medium"/>
      </MultiTrigger.Conditions>
      <MultiTrigger.Setters>
        <Setter Property="Foreground" Value="{DynamicResource CheckedBrush}"/>
      </MultiTrigger.Setters>
    </MultiTrigger>
  </Style.Triggers>
</Style>