This article applies to Nelio A/B testing versions prior to 5.0.
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Each alternative of a page (or post) is actually a full page (or post) itself. As such, its ID and the original's are different. Many people create Advanced Custom Fields (ACF) that only apply to specific pages or posts, based on their ID. Make sure the ACF fields you're missing do not have this limitation and, if they do, add the alternative's ID to their definition.