possibily fixed 3780, adjusted the demo app to play around and reproduce the bug

Author: corvinsz

src/MainDemo.Wpf/NumericUpDown.xaml

@@ -35,7 +35,11 @@
       <smtx:XamlDisplay Margin="10,5"
                   VerticalAlignment="Top"
                   UniqueKey="decimalUpDown_default">
-        <materialDesign:DecimalUpDown ValueStep="0.5" Minimum="-2.5" Maximum="2.5" />
+        <StackPanel>
+          <materialDesign:DecimalUpDown Value="{Binding MyDecimal, Mode=TwoWay}" ValueStep="0.5" Minimum="-2.5" Maximum="2.5" />
+          <TextBlock Text="{Binding MyDecimal, Mode=TwoWay}" />
+          <Button Content="a button to focus"/>
+        </StackPanel>
       </smtx:XamlDisplay>
 
       <smtx:XamlDisplay Margin="10,5"

src/MainDemo.Wpf/NumericUpDown.xaml.cs

@@ -1,7 +1,18 @@
-namespace MaterialDesignDemo;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace MaterialDesignDemo;
 
 public partial class NumericUpDown : UserControl
 {
-    public NumericUpDown() => InitializeComponent();
+    public NumericUpDown()
+    {
+        InitializeComponent();
+        this.DataContext = new MyVM();
+    }
+}
 
+public partial class MyVM : ObservableObject
+{
+    [ObservableProperty]
+    private decimal _myDecimal;
 }

src/MaterialDesignThemes.Wpf/UpDownBase.cs

@@ -213,12 +213,9 @@ private void OnTextBoxFocusLost(object sender, EventArgs e)
         {
             if (TryParse(textBoxField.Text, CultureInfo.CurrentUICulture, out T? value))
             {
-                SetCurrentValue(ValueProperty, value);
-            }
-            else
-            {
-                textBoxField.Text = Value?.ToString();
+                SetCurrentValue(ValueProperty, CoerceNumericValue(this, value));
             }
+            textBoxField.Text = Value?.ToString();
         }
     }