WPF - Storyboard Completed Event not firing in XAML

as the name suggests, I've been trying to use the Completed event from the Storyboard class to do stuff when animations finish, but it seems like the event is never called, despite my bindings being correct.

                                        <MultiDataTrigger>
                                        <MultiDataTrigger.Conditions >
                                            <Condition Binding="{Binding IsMouseOver, ElementName=MyToggle}" Value="True"/>
                                            <Condition Binding="{Binding ShouldOpen}" Value="True"/>
                                        </MultiDataTrigger.Conditions>

                                        <MultiDataTrigger.EnterActions>
                                            <BeginStoryboard x:Name="toto">
                                                <Storyboard Completed="Storyboard_Completed">
                                                    <DoubleAnimation Storyboard.TargetName="MyGrid"
                                                                 Storyboard.TargetProperty="Height"
                                                                 Duration="00:00:01"
                                                                 From="200"
                                                                 To="300"/>
                                                </Storyboard>
                                            </BeginStoryboard>
                                        </MultiDataTrigger.EnterActions>
                                    </MultiDataTrigger>

What I expect : For the method in code-behind named "Storyboard_Completed" to be called after the DoubleAnimation ends.

What happens: The DoubleAnimation does play, but the method is never called. Messing with the conditions (commenting and uncommenting them, changing the value,...) to cause binding failures does make it so that my Storyboard_Completed method is sometimes called, showing that the binding seems to be correct.

I'm trying to figure what I did wrong here, any help is greatly appreciated!