jamesdumay nope this is probably something that falls into the "interesting" category. Some users would like to classify the destination changing as making all the PR-merge heads having an change interesting enough to trigger a corresponding build storm... though perhaps not as interesting as a change to the PR HEAD
Other people do not want the build storm ever.
Until JENKINS-45502 we probably cannot make either camp completely happy... right now you get the build storm with every periodic scan, but not with the event driven builds (because that at least lets both camps have some control through the frequency of scanning)
Note that this behaviour wouldn't work for everyone, because it can trigger build storms:
JENKINS-37491.