A recent PR:
triggered a corresponding build, as anticipated:
This build detected no problems: the PR merged successfully, the corresponding code compiled cleanly and the downstream jobs (which deploy the build and run functional tests) all passed.
However, when the pull-request (along with several others) was accepted, the build failed. Analysis of the problem showed that this pull-request alone was responsible for the problem and that, if the pull request was actually testing, the build could not have succeeded – the problem is also manually reproducible: creating a branch from the latest tagged release and manually cherry-picking the corresponding patch results in code that does not build.
Therefore, for whatever reason, the ghprb plugin is not testing how the code will look after accepting the pull request.