Hm, I thought the idea is that the only way something will land in master is if it reviews OK and CI passes all the tests on top of the current master?
Reviewer comes and approves PR#1, it is merged into master. Depending how much CI power you have, PR#2 (and any other PR in queue) can be automatically built again on top of new master to verify it wont break anything.
You can do it lazily too and just re-check PR#2 once it gets approved by human - like other comment mentioned, CI coordinates and serializes all merges into master making sure master is always green.
And then, there is the "I merged a fix into the release branch, which auto-merge into master, the fix built and tested OK in the release branch but not when propagated to master".