Ah, thanks so much for pointing this out! I just did some research and discovered that the CSS vh unit that I’m using for that feature has buggy support in iOS 6.1+ browsers: http://caniuse.com/#feat=viewport-units

For now, I recommend NOT using the Overlay Post Title on Post Cover option (Dashboard → Appearance → Customize → General Options). If you use Full-Width Featured Images on posts without that option, it won’t use the CSS vh units.

I’ve opened a bug report for this here: https://github.com/raamdev/independent-publisher/issues/150