I found similar (or at least is sounds like) problem within other threads but with no solution.
I work on this website: http://beerprague.webflow.io/. I think there is a bug on mobile in lightbox (iPhone 6s iOS 10, Safari, Chrome, FireFox).
Scenario to reproduce:
1) load the page on mobile
2) open any picture in ligtbox (there are four pictures app in the middle of the homepage). Should look like this:
3) change orientation (you can also close lightbox, change orientation and open it again)
4) you should see something like this:
5) reload page in new orientation and open ligtbox - it is fixed. Until you change orinetation again
It doesn't matter whether you start in portrait or landscape.
So I think there is a bug in recalculating lightbox image position when orientation changed. I wasn't able to reproduce this on desktop by resizing browser's window. Or am I wrong?