Perhaps because contacts and opportunities do not have a parent-child relationship, all child records are merged with the parent during the merge process.
If the contact already has a parent-child relationship with Opportunity and it still doesn't merge properly,
we recommend that you raise a support ticket to give Microsoft feedback on this issue:
If my answer was helpful, please click Like, and if it solved your problem, please mark it as verified to help other community members find more. Please feel free to contact me if you have any further questions. Regards, Haig
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