backgrounding a process doesn't return an exit code (returns null)

[chrisfromredfin]

Steps to reproduce

Created a task called watch that should fork a processes to the background.

public function watch() {
   // run bundle exec compass watch in background
  $res1 = $this->taskExec('bundle exec compass watch --poll')
    ->dir('sites/all/themes/fpuu')
    ->background()
    ->run();
}

Expected behavior

Should send the process to the background and return exit code 0.

Actual behavior

The exit code returned is null, and the bundle exec process is not present in the process table. However a plain old "compass watch" process is running.

System Configuration

I'm using Ubuntu 14.04 in a VM and php 5.6.30 with the Sury repo:
PHP 5.6.30-10+deb.sury.org~trusty+2 (cli)

I think this has something to do with how Bundler deals with sending processes to the background. However, when I manually run that command (bundle exec compass watch --poll &) and then get the status code, it is 0.

[mancioshell]

same here

[vitoo]

Same behavior on
Fedora 28
PHP 7.2.7