Why is broccoli-sass-source-maps looking in the wrong directory for my SCSS file?

I'm using broccoli-sass-source-maps with an Ember 2.18.2 application. (In the process of upgrading from an even older version of Ember.js). When I try to build the application, I get this error:

Build failed.
Build Error (Funnel)

ENOENT: no such file or directory, lstat '/[PROJECT HOME DIR]/tmp/funnel-input_base_path-jVWRZW6Y.tmp/images/styles/temporary.scss'

The actual location of the file is /[PROJECT HOME DIR]/app/underway/styles/temporary.scss. Therefore, in our ember-cli-build.js file, we have configuration like this (I've removed a lot of stuff which seems unrelated to me):

let compileSass = require('broccoli-sass-source-maps')(require('sass'));
let mergeTrees = require('broccoli-merge-trees');

let underwayCss = compileSass(['app/underway/styles'],
  'temporary.scss',
  'assets/temporary.css');

// Merging all the assets together to be served
let assets = mergeTrees([
  underwayCss,
  // output of several other trees
], {
  overwrite: true
});

return app.toTree(assets);

Why is the compileSass() method trying to find the temporary.scss file in an images/styles directory?