I experienced a similar problem, so my solution may help you. In short, it has to do with the URL bar built into Mac mobile device browsers. You can read a better description at this person's page (https://medium.com/@susiekim9/how-to-compensate-for-the-ios-viewport-unit-bug-46e78d54af0d). In short, this "feature" built into Mac devices is a major pain to developers since it dynamically changes the bar's height whenever someone scrolls up and down. I found this the most useful fix, as long as you are trained in JavaScript or CSS: https://css-tricks.com/the-trick-to-viewport-units-on-mobile/.