This isn’t a huge bug because on mobile I typically use the unofficial mobile app but I figured I’d report it because it’s pretty obvious (and probably an easy fix).
On iOS, Safari won’t load map tiles in the bottom half of the viewport. It isn’t until I drag any small portion of the unloaded area into the top half of the viewport that it loads the tiles. This is about the furthest I can drag the map up before it’ll actually load tiles on the bottom half of the screen (about 400px loaded, 440px unloaded):
