When the latest iPhone update iOS 11.4 came out, the images in the eblasts I was creating were suddenly not showing up when viewed by iPhone mail. It only affected the images where -webkit-mask-image was being used. After many hours of frustration playing with the code, testing different things, I finally came across a solution. So, in case anyone else out there comes across this very specific situation, just by using -webkit-mask-box-image instead solved my issue.

Hope that helps out someone.