This JS code isn't running after brunch deploy -p

I would try a more specific path like this:

joinTo: {
  //'js/app.js',
  'js/app.js': /^(js|node_modules)/,
  'js/grants.js': /^(js\/grants.js)|(node_modules)/,
},

I think just doing ^(grants|node_modules) might be trying to match on a folder called grants or something similar.