I am using the latest versions of ViewPoint and PhotoLab. I select a picture of a ruined tower and apply the rectangle correction so as to straighten the sides of the tower but keep the ground horizontal. On applying the transform, the top of the image, including the top of the tower is cropped. There is no obvious cropping at the bottom. The saved image is similarly cropped.
This is normal, PhotoLab does not create pixel.
I think if you set the crop tool/mode to automatic based on keystoning with unconstrained aspect ratio, you will see a better result.
There will always be some cropping necessary when using Viewpoint. since it distorts the shape of the image and needs to be cropped to maintain parallel sides. The greater the amount of adjustment you apply, the greater the amount of cropping that may be necessary. It is pretty similar in that regard to other perspective tools, like Lightroom’s Transform.
As @Egregius indicated, you also need to ensure that the Crop tool Correction setting in the Geometry pallet is set to Auto based on keystoning/horizon which automatically constrains the cropping based on the perspective change. If you don’t, you will end up with missing parts of the image near the edges, a parallelogram, or worse, instead of a rectangle. You should also try just using the parallel perspective tool. It should only affect the verticals of your tower and leave the ground horizontal, assuming it already was.
Adding perspective changes is often more effective on images where the main subjects to be straightened have sufficient space around them. When they don’t, as in your case, there is the possibility that part of that main subject could be cropped a bit. How much of the image is cropped depends on which perspective tool is used, where in the image it’s applied, and how much the perspective is modified.
The tool, as @Pieloe mentioned, does not add pixels to keep all the sides parallel after perspective is applied. More experience with using the tool will help you minimize cropping of important subjects. Unfortunately, it is sometimes inevitable, especially if the subject to be straightened needs a lot of adjustment and is near the image boundaries.