I just tried to use the latest update of FilmLab (on an iPhone SE) with a negative image from Wikimedia Commons and it worked pretty well.
Here is a link to the image on Wikimedia Commons: https://upload.wikimedia.org/wikipedia/commons/e/e7/Big_Ben_Negative.jpg
here is the result:
Here is a link to a PDF that shows the “before” and “after” image
It was a little finicky (being the beta and all), but I helped it along by putting a white border around the image as shown in the file. I had to try it a number of times before it finally worked, but I did get it.