We've had to disable the "Reset approvals when source branch is modified" merge check as it was generating a great deal of noise in some of our projects (because reasons, we've raised other issues ).

So, the suggestion I have is that the PR's author should be able to identify the need to reset approvals and request reviewers re-review, regardless of the settings applied.

So the flow would be:

1. author raises PR
2. reviewer approves
3. author or another approver realises they need to make a substantial change
4. substantial change is pushed to PR
5. (here's the suggestion) author hits "Request re-review" kind of button
6. approvals are reset and reviewers notified of the request

There are workarounds like declining the whole PR and raising a fresh one; but this would be a nicely nuanced option. I've noticed people are relatively uncomfortable declining PRs but they are quite happy to update and at-mention people requesting re-review.