Material Design WPF TextBox Validation error message change position when scrolling page

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I’m new in Material Design. Could someone, please, explain me how can I fix a bug with validation message in textbox
enter image description here

Validation error shows uncorrectly – changes position when I’m scrolling view.
Here is code from .xaml

<TextBox Grid.Row="11"
                 Grid.Column="1"
                 Width="20"
                 VerticalAlignment="Center"
                 materialDesign:ValidationAssist.UsePopup="True"
                 materialDesign:ValidationAssist.OnlyShowOnFocus="True"
                 HorizontalAlignment="Left"
                 ToolTip="Use a popup which can escape the bounds of the control where space is limited">
            <TextBox.Text>
                <Binding Path="Name"
                         UpdateSourceTrigger="PropertyChanged">
                    <Binding.ValidationRules>
                        <helpers:NotEmptyValidationRule ValidatesOnTargetUpdated="True" />
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>