iOS/iPadOS 13 broke the scaling routine in some versions of Video Physics. You could not alter the default 1.0 m scale factor. In the Origin & Scale setting, you can’t alter the contents of the distance and unit boxes.
If you are using a meter stick to scale the video, you can still scale the video correctly. But, if your scale factor is something else, you cannot set the scale.
This is fixed in the version 3.1.2 of the app, which came out on March 16th, 2020. If you are seeing this problem, go to the app store and update the app.