Fieldset border is missing in iphone6 plus

I am working on the responsive part of my website, and in iphone6 plus, fieldset borders are missing(both portrait and landscape mode).

i checked fieldset (in w3schools site) on my iphone6 plus, In portrait mode i can see border but it disappears on landscape mode.

For all the devices iphone and ipad just zoom in and fieldset border will disappear.

If you remove legend, border will appear again in all the cases.

Please provide some pointers