I'm experiencing the same thing.
Doesn't work in development or when built. Could it have something to do with an external pipeline?
My config looks like:
# Pretty links
# Syntax highlighting
set :http_prefix, '/docs'
# Asset pipeline
command: build? ?
'./node_modules/webpack/bin/webpack.js --bail -p' :
'./node_modules/webpack/bin/webpack.js --watch -d --progress --color',
configure :build do
(Yes, that<s it)