I observed issues with drag-and-drop question types in Moodle quizzes also in older Moodle versions, but some seem to have been fixed (for example MDL-62239).
Now on Moodle 3.9, I compared identical courses/quizzes with Moodle 3.5.1 on the same iOS devices, and drag-and-drop clearly works much better in 3.5.1. There I could use these questions even on a small display (iPhone X). In 3.9, the same questions don't work at all on the same iPhone X. On iPads, it's mixed, like all the questions work on a iPad Pro 12.9" 3rd generation, but on a iPad Pro 11", they only work when the display is held in portrait, not in landscape.
The latter case is very weird, is Moodle 3.9 using screen sizes (resolutions) to determine if running on a mobile or desktop browser? That seems the only reason to me why drag-and-drop works in portrait and not in landscape (in both cases the page doesn't fit the height of the display, so it doesn't seem to be directly connected to a page scrolling conflict).
Anybody observed something similar? I can post testing links.