A readable iPhone backup is not automatically a complete message history.

Messages in iCloud adds nuance

Apple explains that data already syncing to iCloud may be handled differently by a computer backup. Third-party tools also document cases where message text remains available while older attachments have been offloaded. Treat these as source-specific possibilities, not a single universal rule. Record the device and sync state, then verify the items that matter.

Build a mixed-source handoff carefully

• Do not merge backup, screenshot, and direct-export content without recording which source produced each part.
• Keep the untouched backup and original captures.
• Use screenshots or screen recording only for missing visible content you are authorized to collect.
• Label unavailable attachments and uncertain dates rather than filling gaps from memory.
• Ask a forensic specialist before changing a device when collection method or authenticity may be disputed.