in iOS, dropdown menus not dropping down in full size mode

Hi there. I’ve got three sites running Suri; two as a child theme, and one test site with Suri as the default (no child theme).

On all three sites, if viewed on an iPad in landscape mode, the dropdown menus don’t drop down. The topmost link in the navbar is clickable, but other menu items don’t appear. In portrait orientation, the mobile layout appears and all is well.

I’ve had this tested on three different devices, two with iOS 10.1.1 and one with 10.2 and the issue is the same on all three.

Your help would be appreciated!