I was running into the same issue and my issue was using x:Bind.

I needed to use Binding and change the Mode:

<TextBlock Name="avgTimeDisplay" Text="{Binding AvgTime, Mode=TwoWay}" Visibility="{Binding LabelVisible, Mode=TwoWay}" ></TextBlock>

This along with setting my ViewModel:

public string AvgTime
{
    get => avgTime;
    set => SetProperty(ref avgTime, value);
}