Rotated video cropped with black borders

I've made my first videos with a new GoPro 9. Due to operator error, a lot of the clips were rotated.
Not a big issue, Resolve has a nice rotation command. So I straightened them all out.

But the sections where I had to use Resolve to correct the rotation are not full screen.
Even in the rendered videos, there is a black mask around the images. Its like it was cropped.

Attached is a screenshot of the video playing in the standard Windows video player.

Did I do something wrong?
Is there a way to correct this in Resolve?

(I wouldn't trust an external player.)

For my own understanding, you're saying you used Rotation Angle of the Transform controls? Or you used one of the Flip options?

Did you use the rotation in the Inspector? I would right-click the clips and choose Clip Attributes before adding them to the timeline and rotate them there. If you used the Inspector Transform rotation you might also have to zoom in.

How did it look in the viewer in Resolve?
Select "Fit" in the top left corner to make sure you're seeing everything.

For my own understanding, you're saying you used Rotation Angle of the Transform controls? Or you used one of the Flip options?

Yes, I used rotation angle in transform
Where are the flip options?

Did you use the rotation in the Inspector? I would right-click the clips and choose Clip Attributes before adding them to the timeline and rotate them there. If you used the Inspector Transform rotation you might also have to zoom in.

Don't I have to put the clips in the timeline to see if they are rotated?

I did zoom in while in Edit, but when they were rendered, they were cropped.

View -> Zoom to fit
zooms to have the black border full size, with the real video a small portion, like in the photo in the first post.

If I click on the zoom percent in the upper left of the viewport, going to 100% shows only the real video, but I think/believe that this controls only the current active viewing and editing session and not what is rendered.

You have to zoom from the Inspector, not the viewer.
If you double-click on a clip in the Media Pool, it will open in the Source viewer and you can see if it is rotated before adding it to the timeline.

Thanks.
I found a setting in my GoPro 9 to keep the image locked in rotation.
I guess most GoPro buyers don't use cool editing tools like Resolve.