> npm ci with node 11 does this:

node-sass@4.11.0 install /home/travis/build/[secure]/[secure]2/node_modules/node-sass
> node scripts/install.js
Downloading binary from https://github.com/sass/node-sass/releases/download/v4.11.0/linux-x64-67_binding.node
Download complete

> npm ci with node 12 does this:
node-sass@4.11.0 install /home/travis/build/[secure]/[secure]2/node_modules/node-sass
> node scripts/install.js
Downloading binary from https://github.com/sass/node-sass/releases/download/v4.11.0/linux-x64-72_binding.node
Cannot download "https://github.com/sass/node-sass/releases/download/v4.11.0/linux-x64-72_binding.node":
HTTP error 404 Not Found

Main difference is the name of the file being downloaded: For node 11 it uses 67 version and for node 12 it uses 72 that is not available yet.