MonoGame.Extended: BoxingViewportAdapter issue with RenderTarget

Game1.cs

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using MonoGame.Extended.BitmapFonts;
using MonoGame.Extended.ViewportAdapters;

namespace Demo.ViewportAdapters
{
    public class Game1 : Game
    {
        private GraphicsDeviceManager _graphics;
        private Texture2D _backgroundTexture;
        private BitmapFont _bitmapFont;
        private ViewportAdapter _viewportAdapter;
        private SpriteBatch _spriteBatch;
        private Point _virtualResolution;
        private RenderTarget2D _renderTarget;

        public Game1()
        {
            _graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
            IsMouseVisible = true;
            Window.AllowUserResizing = true;

            _virtualResolution = new Point(720, 1280);
            _graphics.PreferredBackBufferWidth = _virtualResolution.X;
            _graphics.PreferredBackBufferHeight = _virtualResolution.Y;
        }

        protected override void Initialize()
        {
            base.Initialize();

            _viewportAdapter = new BoxingViewportAdapter(Window, GraphicsDevice, _virtualResolution.X, _virtualResolution.Y);

            var pp = GraphicsDevice.PresentationParameters;

            _renderTarget = new RenderTarget2D(
                GraphicsDevice, 32, 32, false,
                pp.BackBufferFormat, pp.DepthStencilFormat, pp.MultiSampleCount, RenderTargetUsage.DiscardContents, true
            );
        }

        protected override void LoadContent()
        {
            _spriteBatch = new SpriteBatch(GraphicsDevice);
            _backgroundTexture = Content.Load<Texture2D>("vignette");
            _bitmapFont = Content.Load<BitmapFont>("montserrat-32");
        }

        protected override void Draw(GameTime gameTime)
        {
            _graphics.GraphicsDevice.Clear(Color.CornflowerBlue);

            // Clear the render target
            _graphics.GraphicsDevice.SetRenderTarget(_renderTarget);
            _graphics.GraphicsDevice.Clear(Color.Red);
            _graphics.GraphicsDevice.SetRenderTarget(null);

            var destinationRectangle = new Rectangle(0, 0, _virtualResolution.X, _virtualResolution.Y);

            _spriteBatch.Begin(transformMatrix: _viewportAdapter.GetScaleMatrix());

            _spriteBatch.Draw(_backgroundTexture, destinationRectangle, Color.White);
            _spriteBatch.DrawString(_bitmapFont, $"Current: {_viewportAdapter.GetType().Name}", new Vector2(49, 40 + _bitmapFont.LineHeight * 4), Color.Black);
            _spriteBatch.DrawString(_bitmapFont, @"Try resizing the window", new Vector2(49, 40 + _bitmapFont.LineHeight * 6), Color.Black);

            _spriteBatch.End();

            base.Draw(gameTime);
        }
    }
}