ComboBox text is not visible when the control is inside Grid.Row="40" and testing on device with 800x1200 mdpi:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="40"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="20"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<telerikInput:RadComboBox BackgroundColor="Yellow" Padding="0"
Margin="0"
Grid.Column="2" x:Name="combo"
Grid.Row="0"/>
</Grid>
Workaround: set higher grid row height: 60 or above, or using Auto row height.