Fieldset border is missing in iphone6 plus

I am working on the responsive part of my website, and in iPhone 6 plus, fieldset borders are missing(both portrait and landscape mode).

I checked fieldset elements on other responsive site(w3schools) on iPhone 6 plus, In portrait mode i can see border but it disappears in landscape mode.

For all the devices (iPhone and iPad): fieldset border disappears, as soon as I zoom in.

If I remove legend, border will appear again (in all the cases).

Please provide some pointers