It seems that $entity->op for field collection entities is empty in function computed_field_field_prepare_view so _computed_field_compute_value won't run for those fields.

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I am experiencing the same issue with version "7.x-1.0-beta1".
I am not sure why $entity->op is tested, as I can't see where this property is set to 'Preview' in Drupal?

After a bit of debugging, I can't figure out how to test if the entity is being previewed or not in this hook_field_prepare_view().
I had to manually set my node property "op" to "Preview" on a beforehand hook to make this work... which seems really hacky...