Fixes spectreconsole#1472

Author: jbennink

src/Spectre.Console/Live/Progress/ProgressTask.cs

@@ -201,6 +201,7 @@ private void Update(
             if (_value > _maxValue)
             {
                 _value = _maxValue;
+                StopTime ??= DateTime.Now;
             }
 
             var timestamp = DateTime.Now;