Compiled SASS from django-pipeline not found by staticfiles even though it's present

Question

I've configured django-pipeline as follows:

PIPELINE = {
    'PIPELINE_ENABLED': True,
    'COMPILERS': (
        'pipeline.compilers.sass.SASSCompiler',
    ),
    'STYLESHEETS': {
        'common': {
            'source_filenames': (
                'sass/common/styles.scss',
            ),
            'output_filename': 'css/common/styles.css',
        }
    }
}

I ran collectstatic and started the server in debug mode to see if everything's set up properly. I can access all my images and scripts, but not the compiled CSS file. My static settings:

STATIC_ROOT = os.environ.get('DJANGO_STATIC_ROOT', os.path.join(BASE_DIR, '_static'))
STATIC_URL = '/static/'
STATICFILES_STORAGE = 'pipeline.storage.PipelineCachedStorage'
STATICFILES_FINDERS = (
    'django.contrib.staticfiles.finders.FileSystemFinder',
    'django.contrib.staticfiles.finders.AppDirectoriesFinder',
    'pipeline.finders.PipelineFinder',
)

Some more observations:

• I added a breakpoint to django.views.static.serve and I noticed that it would trigger for http://127.0.0.1:8000/static/sass/, but not for http://127.0.0.1:8000/static/css/ or http://127.0.0.1:8000/static/css/common/styles.css. However, the compiled CSS file is present in _static/css/common/ (as styles.css and styles.cba140b2c4b2.css).
• If I copy the contents of _static/css/ into the originating app's static folder, the CSS renders normally.
• Another possible symptom is that my templates aren't outputting versioned static file paths.

How can I make staticfiles find the compiled CSS file?