Simple solution. Thanks to the help of @nicodemus on the Discord channel. This break in the expected behavior of the development framework is due to the upgrade to Webpack 1.5. In order to get hot reloading again, you need to add the --watch flag to the watchers entry in dev.exs. The flags changed from v1.4 to 1.5.

Here’s an example (lines 11 and 12):

 1 config :nexus, App.Endpoint,
 2  http: [port: 4000],
 3  debug_errors: true,
 4  code_reloader: true,
 5  check_origin: false,
 6  watchers: [
 7    node: [
 8      "node_modules/webpack/bin/webpack.js",
 9      "--mode",
10      "development",
11      "--watch",  # ADD THIS
12      "--watch-options-stdin", # THIS FLAG ALSO CHANGED IN 1.5
13      cd: Path.expand("../assets", __DIR__)
14    ]
15  ]