When a merge task is created and merge is performed it is expected that the task will be completed and there is no final review after merge task. you could create a final review before merge task but on an ideal use case, the merge task should be assigned to the same user who is going to perform the final review.
This behavior should not be different from MDM 971.
We have checked with Informatica and they have confirmed it as bug in earlier versions prior to 10.2 and fixed in current version. Prior versions shows final review tasks even after merging 2 customers. This is true in the case when task is created for one record.