When a pull request is updated, the approvals remain. In the Confluence dev team, we're currently using the auto-unapprove plugin that clears out the approvals. This isn't ideal either, because it isn't obvious from looking at the review that the reviewer ever approved the change.
What would be ideal from my perspective as a team lead is if Stash indicated approvals that were made on an earlier version of a pull request with a different or greyed-out approval icon. In the tooltip for the approver, it could also say "Approved on an older version of this pull request."
If that isn't clear, let me know and I'll mock up an example of what I'm suggesting.