Cannot update instance metadata

[pegasd]

Going from

gcompute_instance { 'my-instance':
  metadata => join([ '#!/bin/bash', "echo 'Hi!'" ], "\n"),
  [ ... snip ... ]
}

to

gcompute_instance { 'my-instance':
  metadata => join([ '#!/bin/bash', "echo 'Hi!'", "echo 'Hi again!'" ], "\n"),
  [ ... snip ... ]
}

produces the following output every time I apply the manifest:

Notice: Compiled catalog for localhost in environment production in 0.65 seconds
Notice: /Stage[main]/Main/Gcompute_instance[my-instance]/metadata: metadata changed {
  'startup-script' => "#!/usr/bin/bash\necho 'Hi!'"
} to {
  'startup-script' => "#!/usr/bin/bash\necho 'Hi!'\necho 'Hi again!'"
}
Notice: Applied catalog in 6.29 seconds

Instance metadata does not actually get updated.