Combobox binding breaks after button click WPF MVVM

Combobox binding breaks after button click wpf mvvm. I have used interaction trigger to bind combobox selection changed to an ICommand.

 <ComboBox  IsEditable="True" Text="Please Select" IsReadOnly="True" ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem}">
                    <i:Interaction.Triggers>
                        <i:EventTrigger EventName="SelectionChanged">
                            <i:InvokeCommandAction Command="{Binding SelectionChangedCommand}" />
                        </i:EventTrigger>
                    </i:Interaction.Triggers>
                </ComboBox>
 <Button Content="Save"  Command="{Binding SaveCommand}" />

Binding works fine till save button's command is fired. Once save command of button is fired, combobox selectionchange command breaks

Edit: Items is a list of string like below

           "A1", "A2", "A3", "A4", "A5", "A6", "A7", "A8", 
           "B1", "B2", "B3", "B4", "B5", "B6", "B7", "B8", 
           "C1", "C2", "C3", "C4", "C5", "C6", "C7", "C8", 
           "D1", "D2", "D3", "D4", "D5", "D6", "D7", "D8"

Selected item is displayed in a readonly textbox and when the alphabet changes, SelectionChangedCommand binding breaks (SelectedItem is no longer updated but, on clicking save, its updated).

Note that Combobox's SelectedItem is bound to below property of ViewModel.

 private string _SelectedItem;
            public string SelectedItem
            {
                get
                {
                    return _SelectedItem;
                }
                set
                {
                    if (_SelectedItem!= value)
                    {
                        _SelectedItem= value;
                        RaisePropertyChanged(() => SelectedItem);
                    }
                }
            }