This is expected behavior in Salesforce Classic Mail Merge. When generating a mail merge from an Opportunity, the merge engine cannot traverse multiple object relationships (Opportunity → Contact → Account) to retrieve the Contact's Account Name - this is called a cross-object reference limitation.
As a workaround, create a formula text field on the Contact object using the formula Account.Name, then reference that field in your Mail Merge template.
See Also:
Use the Salesforce Merge Language in Classic Email Templates
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