iA Writer Duo works as expected in Drafts. However, iA Writer Quattro always defaults in Drafts to its bold variant, even though all four variants (Regular, Italic, Bold, and Bold Italic) are verifiably installed by the Fontcase profile.
This is a minor issue, of course. But iA Writer Quattro is designed for the limited screen width of smaller devices, making it a nice custom font choice for Drafts on iPhone. If this is indeed a Drafts issue and not an iOS issue, it would be lovely to see it fixed.