I also had same problem and I solved like below. I had to go through a lot of forums including this but none works. But some forum asked me to forcibly load font before I come to point of my interest.
In my TextBlockStyles.xaml resource dictionary I created a styke
<Style x:Key="RadWindowTitleTextStyle"
TargetType="TextBlock">
<Setter Property="Foreground"
Value="#FFB9D449" />
<Setter Property="FontSize"
Value="18.667" />
<Setter Property="FontFamily"
Value="/sampleawebsite.application;component/Assets/Fonts/UBSHead.ttf#UBSHeadline" />
</Style>
In my MainPage, I just gave below to load my font
<TextBlock Text="Do not display Font load purpose only"
Width="1"
Height="1"
Style="{StaticResource RadWindowTitleTextStyle}"
Foreground="Transparent" />
Below is my point of interest in one of my ChildWindow popup styles resource dictionary called RadWindowStyles.xaml
<TextBlock Text="{TemplateBinding Title}"
Style="{StaticResource RadWindowTitleTextStyle}" />