Currently, when you approve an existing pull request and there is a simultaneous commits pushed against the PR, the reviewer will likely miss the new commits/files to be reviewed due to this race condition.
By default, the PR will not reset the approval, however, even if you use the "Reset approvals" branch restriction setting, there will still be confusion and/or time waste in reviewing the PR where it has to be reviewed again.

Enhancement request:
It would be useful to improve the functionality while approving a pull request to provide a validation if a PR is updated or there are new commits to the PR before proceeding to approve the PR, this will warn the approver to review the PR changes again.
Additionally, it would be a nice feature to be able to review only the recently added commits/files as well.