Value not updated after property changes on `DependencyObject` + `INotifyPropertyChanged`

I am binding to an object, that implements both `DependencyObject` and `INotifyPropertyChanged`. The property only has a getter.

This works and gets updated as usual:
```xaml
<Run Text="{Binding PowerLimitMilliwatts, Mode=OneWay}"/>
```
but this does not get updated:
```xaml
<Run Text="{calc:Binding PowerLimitMilliwatts/1000, Mode=OneWay}"/>
```