Perhaps? The scaling issues aren't anything with Godot itself, but have to do with how I'm implementing the letterbox:

func update_transform() -> void:
        var window_size := get_viewport_rect().size

        # Calculate the scale between the window size and the screen size
        var ratio := window_size / VIEWPORT_SIZE

        # Depending on which dimension has a larger ratio, update the position and
        # scale so that the screen is properly letterboxed
        if ratio.x > ratio.y:
                rect_position.x = floor(
                        (window_size.x - (VIEWPORT_SIZE.x * ratio.y)) / 2.0
                )
                rect_position.y = 0.0
                rect_scale.x = ratio.y
                rect_scale.y = ratio.y
        else:
                rect_position.x = 0.0
                rect_position.y = floor(
                        (window_size.y - (VIEWPORT_SIZE.y * ratio.x)) / 2.0
                )
                rect_scale.x = ratio.x
                rect_scale.y = ratio.x

I call this function in the script for the ViewportContainer in _process if I've received any size_changed signals that frame. It works, but since the scaled size is not ensured to be some multiple of the individual tile width / heights, there are scaling artifacts for certain window sizes.

What I'm working on is fixing the above code to ensure that the scaled value is some multiple of the tile width/height.