"Expression is always false" in pattern only when #nullable enable is off

I have a .NET Standard 2.1 project (thus it uses C# 8) but have the project-level setting for reference type nullability as off. I turn it on for select files in that project with #nullable enable.

If I have code with try/catch that uses pattern matching, matching a more derived type after checking for a broader type underlines the former with "Expression is always false" but only in the files that I haven't turned on nullability.

This block has the "e is TaskCanceledException" underlined with the "expression is always false" message (file has nullability off), but if all I do is turn on #nullable enable for the file, the underline goes away.

try
{
   // some code that could throw an OperationCanceledException or a TaskCanceledException
}
catch (Exception e) when (e is OperationCanceledException || e is TaskCanceledException)
{
  Logger.Trace($"In {nameof(ShowMenu)}: A task was canceled.");
}