On the other hand, the autoprefixer.js from the gem (about 1 MB) and the autoprefixer.js that's generated through build from the upstream repo (about 8 MB) seems to be different. This is unexplained by the dev.
I was comparing an older version of the gem actually. Please ignore this comment.
On the other hand, I have found that the node-resolve is somehow resolving util.js from the system (/usr/share/nodejs) instead of node_modules (possibly related to the patch we're applying) and that causes process.env to be read directly (without handling it as if it is inside browser)
