NumericUpDown Focus doesn't work if set through FocusManager

[gcores]

Trying to set focus to NumericUpDown through FocusManager doesn't work. E.g.:

FocusManager.SetFocusedElement(GridControl, NumericUpDownControl);
<Grid Name="GridControl" FocusManager.FocusedElement="{Binding ElementName=NumericUpDownControl}">

but setting it like this does:

NumericUpDownControl.Focus();

Environment

• MahApps.Metro v1.5.0
• Windows OS 10
• Visual Studio 2017
• .NET Framework 4.7