Why am I getting a casting error in .NET 7 after using reflection?

Is reflection and casting different in .NET 7 vs .NET Framework? I'm porting a project and getting this casting error after moving the code over. The wierd thing is this class implements that interface. This code works in .NET 4.x.

    foreach (Assembly pluginAssembly in pluginAssemblies)
    {
        try
        {
            // Look for class(s) with our interface and construct them
            foreach (var type in pluginAssembly.GetTypes())
            {
                Type iDesigner = type.GetInterface(typeof(IFireworksDesigner).FullName);
                if (iDesigner != null)
                {
                    object instance = Activator.CreateInstance(type); // creates an object 
                    IFireworksDesigner designer  = (IFireworksDesigner)instance; // throws an exception
                    // do stuff
                }
            }
        }
        catch(Exception ex)
        {
            //Something really bad must have happened. 
            MessageBox.Show("Fatal error reflecting plugins in assembly '" + pluginAssembly.FullName + "'.\r\n" +
                "The error message is:\r\n\r\n" + ex.Message);
        }
    }