Can you be more specific about the failure mode here? If we have a series master -> A_review -> B_review, and B_review is targeted to A_review in the PR tool, then a fast-forwarding merge of A_review to master just advances the master branch name and automatically updates B_review to now target master. No merge commits take place.
At least in bitbucket, you can just set a branch under review as the target of a pull request and the UI "just works" for that case.