I used the example from this website, I believe it's created by 
dop2000.
howtoconstructdemos.com/virtual-thumbstick-to-control-a-character-with-8-direction-behaviour-capx
I think a few years back it didn't work on my iPhone either. I tested it on different devices; iPhone 5SE, iPhone 7, and an Android Tablet which is about a 1,5 years old. When I asked someone to test the game (browser based), he said the thumbstick didn't work on his iPhone. It just doesn't move.
The weird thing is, on the Android tablet it always seems to work. It's just the iPhones that give trouble. I tried different browsers like Chrome, Safari, and incognito and normal browser tabs. I got various results.
It does seem work in certain conditions! I can't always 100% replicate it but I'm under the impression that closing all browser tabs and the browser app, opening the browser app again and (re)loading the URL from a non-incognito tab there's a higher chance of the thumbstick working than when initially openened in incognito.
On desktop it always works fine.
Any help's appreciated.