I am also able to reproduce this issue. It seems to affect Apple IOS devices and it doesn't matter what browser you use as Apple forces you to use their webkit web framework in the back-end. This is not an issue on Android.
This issue also affects any pop-up modal wrapped in a Responsive Table DIV for example edit client under client listing.
I'm not sure what the fix is going forward but it seems like an upstream issue with Apples Webkit on IOS devices.