Haven’t used Final Cut in a while. I’ve never used final cut with an anamorphic lens or a Helios fake anamorphic lens. I took some at 3.5 K footage and every time I Exported the video it always came out in a box. Are used letterbox in final cut pro and change the aspect ratio but it still comes out in a box. The test footage is below.
https://youtu.be/GkdRJ2TaB0E