I had the problem of the header image zoomed in and clipping off content even though the photo was scaled to the recommended dimensions. I noticed that the image sits behind the administrator bar at the top of the screen and the menu bar below. Removing either of these if possible may add more visibility. After just blindly hacking away at a solution, I arrived at this solution which seems to work for my needs. I added this to the “additional CSS” option under the “customize” menu:
div#wp-custom-header.wp-custom-header img {height: 100%;object-fit: fill }