Data Missing in CustomTextBox Control(WPF)

Using a CustomTextBox Control, in Hospitality Application.

We are facing some issues inconsistently, like

Data disappear while typing in textbox. Data erased in lost focus.(Moving from one tab to another)

We dug lot in our application and ensured all the functionality, styles handled properly.

Custom Control

            <local:WatermarkedTextBox x:Name="txtboxcomment"                                                                                                           
                           Text="{Binding Comment,UpdateSourceTrigger=Explicit}"                                                                              
                            Style="{StaticResource WatermarkTextBoxExpandable}"                                                                                
                        <interactivity:Interaction.Triggers>
                            <interactivity:EventTrigger EventName="LostFocus">
                                <local:BindingGroupCommitEditAction                                                                          NewValue="{Binding Text, ElementName=txtBoxComment}"                                                                          CurrentValue="{Binding Comment}"      
    Command="{Binding Path=findcommentHandler}"                                                                          CommandParameter="{Binding}"> </local:BindingGroupCommitEditAction>
                            </interactivity:EventTrigger>
                        </interactivity:Interaction.Triggers>
                    </local:WatermarkedTextBox> 

I have update the sample in the below link:

https://drive.google.com/drive/my-drive

Anyone having idea regarding the issue?