I couldn’t find any related topic about this, but when I tried with my install. I’ve found that I only get add to home screen icon on my iPad when I use any URL with sub domain, ie. www.example.com, x1.example.com. However, if I use example.com as URL, I will not get the icon at all.
First I thought it was my own issue, URL redirect thing, but as far as I’ve tried, it’s not.
note: my apple_touch_icon_url is on S3.