String.Format(formatProvider, "{0:N}", value) regression

Running the code:

```
var formatProvider = CultureInfo.GetCultureInfo("nl-BE");
Console.WriteLine(String.Format(formatProvider, "{0:N}", 12.3456789m));
Console.WriteLine(String.Format(formatProvider, "{0:N}", 12345.6789m));
```

Will output for .NET Core 2.0 or 3.1 LTS:
12,35
12.345,68

Will output for .NET 5.0 or 6.0:
12,346
12.345,679

Is this intentional?