Input sourcemaps seem to not be honored

I'm running gulp-typescript before piping through gulp-babel. I init sourcemaps before piping through TypeScript. The output source map from TypeScript seems to be ignored by gulp-babel. Babel itself does support input type maps.

Am I doing something wrong or is this a bug?

Reproduce the gulp-typescript to gulp-babel pipeline with sourcemaps initialized before TypeScript, then inspect how gulp-babel handles the resulting input map. Done means determining whether the map is ignored by the plugin or the pipeline is misconfigured, and documenting the result or correcting the behavior.

Written by the indexing model from the issue text.