When there's a default style defined somewhere in resources on the TextBox type like this:
<Style TargetType="TextBox" BasedOn="{StaticResource MaterialDesignFloatingHintTextBox}">
<Setter Property="Height" Value="40" />
</Style>
It will displace the TimePicker because of the TextBox inside the template not having a theme (see image)

To fix the issue, I added following Style property to the ControlTemplate of the TimePicker in MaterialDesignTheme.TimePicker.xaml:
Style="{DynamicResource MaterialDesignTextBoxBase}"
To see the issue first hand, I've set up a test project named MaterialDesignTest: https://github.com/MichelMichels/MaterialDesignInXamlToolkit/tree/TimePickerBug
When there's a default style defined somewhere in resources on the TextBox type like this:
It will displace the TimePicker because of the TextBox inside the template not having a theme (see image)
To fix the issue, I added following Style property to the ControlTemplate of the TimePicker in MaterialDesignTheme.TimePicker.xaml:
Style="{DynamicResource MaterialDesignTextBoxBase}"To see the issue first hand, I've set up a test project named MaterialDesignTest: https://github.com/MichelMichels/MaterialDesignInXamlToolkit/tree/TimePickerBug