Think of it like this. Something like Gmail.com is really two things:

1. a frontend, which is a Single Page App, shipped as a blob of JS;

2. a backend, which is a big fat set of services in random languages.

The people who create the frontend are traditionally thought of as web developers, but "UI/UX engineer" is perhaps closer: they're engineering a program, yes, but it's ultimately a program entirely to serve the purpose of being a nice UI/UX in front of an API.

I think the most on-target name for this role would probably be "client-app developer." The client happens to be web-based, but that doesn't much matter. It's still a client that consumes your service.