Hi tixrus,
Welcome to Apple Support Communities. It sounds like you are trying to crop directly from Photos. Try cropping in iMovie instead to see if that gives you the same result. Guidance below is from Crop clips in iMovie on Mac
"Crop a clip or photo in the timeline
- In the iMovie app on your Mac, select a clip or photo in the timeline.
- To show the cropping controls, click the Cropping button.
- Click the Crop to Fill button.
An adjustable frame appears on top of the clip in the viewer.
Note: This frame is constrained to a 16:9 aspect ratio.
4. Move and resize the frame until you’re satisfied with the result.
5. To apply the crop, click the Apply button in the cropping controls.
The area within the crop frame fills the viewer.
To reset all crop and rotation adjustments, click the Reset button.
If you applied a crop to the clip in the browser before adding the clip to the timeline, that crop is reset as well."
You can also try editing the video from your iPhone. Guidance can be found in How to edit videos on your iPhone, iPad, iPod touch, or Mac.
Take care.