Phone screens are not like printed paper, even if they seem similar to the naked eye.
A scanner for paper is designed to illuminate paper and read the reflection. So the scanner could send a specific wavelength, and expect the same. Phone screen could not deliver such wavelength (it must just have good R, G, B), but pretty free on how to choose the wavelength distribution. Old scanner used in particular deep red or infrared. Phone will not have infrared transmission on white.
Scanner could choose the intensity, but a phone reader not.
Additionally white on paper is continuous, but a phone screen is not, you will have lots of dark surfaces (especially if you filter e.g. only red). It is above our eyes' resolution, so not a problem for us, but a scanner should have a signal filter, not to see such black, but still see the black dots.
In facts, I have had many more problems when scanning tickets on a phone (in general, not only at airports) than on paper.
Modern scanners have a camera and these cameras are much more sensitive (on low screen light), so the problem is disappearing.
So, it is risky. If you have a "retina"-like phone, with many pixels, and a very bright screen, it will be less problematic, but with a cheap phone you may have problems. On the other hand, good phones have better colour (and "LED" pixels), better colours means more saturated colours, so less wavelengths. So it is your risk.