Answer by Liyun Zhang - MSFT for The result of a sum is incorrect when all numbers are deleted from a TextField

I can reproduce your problem and I used the trigger to fix it:

<Entry x:DataType="local:MyViewModel"        x:Name="entry"        Keyboard="Numeric"        Text="{Binding Notificado}"        TextColor="Black" ><Entry.Triggers><DataTrigger TargetType="Entry" Binding="{Binding Source={x:Reference entry}, Path=Text.Length}" Value="0"><Setter Property="Text" Value="0"/></DataTrigger></Entry.Triggers></Entry>