If you uncheck view as sq pix you should be able to see it in your monitor. FCP has an issue with those pesky sq pix. As a rule if it looks ok in the Broadcast Monitor, it doesn’t matter what it looks like in the viewer. Remember TV pixles are non square. fit/fill to window is a good setting, I don’t normally pay attention, I just set it to 100% and non square, that way I know I have no distortions.
hope it helps