The Divi Theme gives subheadings in posts (i.e <h2> tags) a bit of bottom padding (10px) to provide space between them and the text that follows. However, on small device widths, Divi reduces this bottom padding to zero. This means that there is very little visual gap between the bottom of the heading text and the top of the body text that follows. While this reduction in bottom padding only targets narrow device widths, iPhones (due to the "retina" display) are among those devices affected. Here's how to increase the gap between the <h2> subheadings and the paragraph text on mobile devices.
Increasing the Post Subheading Bottom Padding using CSS
Where there isn't a setting in Divi to adjust the default gap below the h2 tags in posts, the following CSS can be used to do so:
As mentioned, Divi sets the bottom padding to zero, so anything larger should increase the space below the subheadings. You may need to tweak the value to get something that looks right to you, but somewhere between 6-10px is probably a good place to start.