The workaround is to unhook the event from within the handler.

I've simplified the steps to reproduce it. It also occurs using the asynchronous BeginRender method.

Save the included script as "C:\Program Files\VEGAS\VEGAS Pro 18.0\Script Menu\RenderFinishedIssue.cs"

1. Copy an mp4 file to "C:\Test\Test.mp4"
2. Open Vegas 18
3. Run the script | Tools | Scripting | RenderFinishedIssue
4. In Vegas 18, two messages are displayed, "completed" and then "rendering"
5. In Vegas 9 through 17, only the first message, "completed" is shown.

using ScriptPortal.Vegas;
using System.Windows.Forms;

public class EntryPoint
{
    public void FromVegas(Vegas vegas)
    {
        const string testMedia = @"C:\Test\Test.mp4";
        const string testRender = @"C:\Test\Rendered.mp4";
        const string testRenderer = "MAGIX AVC/AAC MP4";
        const string testTemplate = "Internet HD 1080p 25 fps";
        
        var videoTrack = new VideoTrack();
        vegas.Project.Tracks.Add(videoTrack);
        videoTrack.AddVideoEvent().AddTake(new Media(testMedia).GetVideoStreamByIndex(0));
        
        var template = vegas.Renderers.FindByName(testRenderer).Templates.FindByName(testTemplate);
        var renderArgs = new RenderArgs(vegas.Project) {RenderTemplate = template, OutputFile = testRender};
        
        vegas.RenderFinished += RenderFinishedHandler;
        vegas.Render(renderArgs);
        vegas.RenderFinished -= RenderFinishedHandler;
        
    }    

    private void RenderFinishedHandler(object sender, RenderStatusEventArgs args)
    {
        MessageBox.Show(args.Status.ToString());        
    }
}