Update: After some testing, I discovered that this issue is specific to iPhones. When I tested it with an android device, everything worked perfectly fine. However, when I tried viewing the page on two different iPhones, it broke on both.
This problem seems to be related to mobile devices, as when I resize my browser window to mimic a mobile screen, everything appears normal. But when I actually view the site on my mobile device, the drop shadow effect "breaks" at random spots. Zooming in and out causes these breaks to move around.
The current CSS effect in use is:
filter: drop-shadow(20px 10px 4px #707070);