After some fiddling around, I have figured out the required syntax for the event in the derived class.  Is there anything else I need to add to the bug report that we need to watch for when doing this?
namespace Test
{
    public delegate void TestEventHandler();
    
    public interface ITestEvent
    {
        event TestEventHandler TestEvent;
    }
    
    public class testEventImplementation : ITestEvent
    {
        // C# 7 version - need to add "public" keyword to this event
        // otherwise the code does not compile
        public event TestEventHandler TestEvent;
    }
}
and from a VS2019 C# .NET core solution:
public interface ITestEvent
{
    public delegate void TestEventHandler();
    public event TestEventHandler TestEvent;
}
    
class TestEventImplementation : ITestEvent
{
    // C# 8 version - keeping public keyword from event in base class
    // need to scope the return type, since the type is not being
    // implemented here, instead used from the base class
    public event ITestEvent.TestEventHandler TestEvent;
}