I'm not 100% sure that this is really the case since my understanding of Dotnet and C# is pretty limited but the current documentation seems to suggest that the problem matchers use EcmaScript-style regex syntax but I don't see the necessary options being passed here: https://github.com/actions/runner/blob/6c70d53eead402ba5d53676d6ed649a04e219c9b/src/Runner.Worker/IssueMatcher.cs#L68
The default seems to be to use canonical regex synatx: https://docs.microsoft.com/en-us/dotnet/standard/base-types/regular-expression-options#default-options