See https://www.phpbb.com/community/viewtopic.php?f=556&t=2490866 for more details. 

We've narrowed it down somewhat and don't think it is dependent on OS or browser, but rather mobile device size. I can reproduce this using Chrome's Inspect Element tool if I set my device to iPhone 5/SE (320px wide) but it works properly on iPhone 6/7/8 (375px wide). On a desktop with Chrome's Inspect Element tool in "Responsive mode", I see the element disappear at 350 px and reappear at 351 px.